M275 – Web Development Using PHP and MySQL

M275 – Web Development Using PHP and MySQL
Arab Open University

The Building Blocks of PHP
Chapter 5 The Building Blocks of PHP

Chapter Outline In this chapter, you will learn:
About variables: what they are, why need them and how to use, define and access them About data types About some the more commonly used operators, how to use them in creating expressions About constants and how to define them

Ch. 5: Variables. Data Types, Operators and Constants
Variables: are special containers for holding certain values of some data types. Data types: are classifications for identifying possible forms of data (e.g. number, string, object, array or Boolean). Operators: are symbols or series of symbols used to manipulate values. Constants: are containers with unchanged remaining values.

Ch. 5: Variables Variables are used whenever the data is liable to change. This happens either within the lifetime of the script, or when it is passed to another script. Ways to assign values to variables: User input. Through database query. As a result of an earlier action in the script.

Ch. 5: Variables In PHP Variable names are preceded by a dollar sign $. Variable names can include letters, numbers and the underscore character “_”. Variable declaration takes place by including the variable in the script. Examples (lab work).

Ch. 5: Variables Rules for declaring and working with variables:
Names should be meaningful and consistent in style. Unless declared global in a script connected with some other scripts, the value of a variable is local to the script it is declared in. (more details in chapter 7). Some variables exist all the time and their values are available to all scripts (Superglobal variables).

Ch. 5: Variables Examples of superglobal variables (are arrays of other variables): $_GET contains any variables provided to a script through the GET method. $_POST contains any variables provided to a script through the POST method. $_COOKIE, $_FILES, $_SERVER, $_REQUEST, $_ENV, $_SESSION (what are these superglobal variables used for?)

Ch. 5: Variables Q: Why do we use superglobal variables?
A: It is crucial in creating secure applications, because they reduce the likelihood of the user-injected input to your scripts.

Ch. 5: Data Types PHP is a loosely typed: no need to declare a type of a variable, as this happens automatically. Advantage: more flexibility. Disadvantage: flexibility has its price in large scripts. For example, when expecting a variable to hold a data type when it holds another.

Ch. 5: Data Types Table 5.1 Standard data types

Ch. 5: Data Types PHP has several functions available to test the validity of a particular type of variable—one for each type, in fact. The is_* family of functions, such as is_bool(), tests whether a given value is a Boolean. You can use PHP’s built-in function gettype() to test the type of any variable. If you place a variable between the parentheses of the function call, gettype() returns a string representing the relevant type.

Ch. 5: Data Types Lab work:
Put the code in Listing 5.1 of the textbook into a text file called testtype.php, and place this file in your web server document root. When you access this script through your web browser, it produces the output:

Ch. 5: Data Types Listing 5.1 Testing the Type of a Variable
1: <?php 2: $testing; // declare without assigning 3: echo "is null? ".is_null($testing); // checks if null 4: echo " "; 5: $testing = 5; 6: echo "is an integer? ".is_int($testing); // checks if integer 7: echo " "; 8: $testing = "five"; 9: echo "is a string? ".is_string($testing); // checks if string 10: echo " "; 11: $testing = 5.024; 12: echo "is a double? ".is_double($testing); // checks if double 13: echo " "; 14: $testing = true; 15: echo "is boolean? ".is_bool($testing); // checks if boolean 16: echo " "; 17: $testing = array('apple', 'orange', 'pear'); 18: echo "is an array? ".is_array($testing); // checks if array 19: echo " "; 20: echo "is numeric? ".is_numeric($testing); // checks if is numeric 21: echo " "; 22: echo "is a resource? ".is_resource($testing); // checks if is a resource 23: echo " "; 24: echo "is an array? ".is_array($testing); // checks if is an array 25: echo " "; 26: ?>

Ch. 5: Data Types The output of Listing 5.1 is:
is null? 1 is an integer? 1 is a string? 1 is a double? 1 is boolean? 1 is an array? 1 is numeric? is a resource? is

Ch. 5: Data Types After the check to see whether $testing is NULL, values are assigned to $testing by using the = sign and then the variable is tested with the appropriate is_* function. An integer, assigned to the $testing variable in line 5, is a whole or real number. A whole number is simply a number without a decimal point. A string, assigned to the $testing variable in line 8, is a collection of characters. When you work with strings in your scripts, they should always be surrounded by double or single quotation marks (" or ').

Ch. 5: Data Types A double, assigned to the $testing variable in line 11, is a floating-point number (that is, a number that includes a decimal point). A Boolean, assigned to the $testing variable in line 14, can have one of two special values: true or false. In line 17, an array is created using the array() function, which you learn more about in Chapter 8, “Working with Arrays.” This particular array contains three items, and the script dutifully reports $testing to have a type of array.

Ch. 5: Data Types From line 20 through the end of the script, no value is reassigned to $testing—only the type is tested. Lines 20 and 22 test whether $testing is a numeric or resource type, respectively, and because it is not, no value is displayed to the user. In line 24, the script tests again to see whether $testing is an array, and because it is, the value of 1 is displayed.

Ch. 5: Changing Type with settype()
PHP also provides the function settype(), which is used to change the type of a variable. To use settype(), you place the variable to change and the type to change it to between the parentheses and separate the elements with a comma, like this: settype($variabletochange, 'new type'); Listing 5.2 converts the value 3.14 (float) to each of the four standard types examined in this chapter.

Lab work: Put these lines into a text file called settype.php and place this file in your web server document root 1: <?php 2: $undecided = 3.14; 3: echo "is ".$undecided." a double? ".is_double($undecided)." "; // double 4: settype($undecided, 'string'); 5: echo "is ".$undecided." a string? ".is_string($undecided)." "; // string 6: settype($undecided, 'integer'); 7: echo "is ".$undecided." an integer? ".is_integer($undecided)." "; // integer 8: settype($undecided, 'double'); 9: echo "is ".$undecided." a double? ".is_double($undecided)." "; // double 10: settype($undecided, 'bool'); 11: echo "is ".$undecided." a boolean? ".is_bool($undecided)." "; // boolean 12: ?> Note: Per the PHP Manual, "double" is returned in case of a float, and not simply "float".

In each case, we use the appropriate is_* function to confirm the new data type and to print the value of the variable $undecided to the browser using echo. When we convert the string "3.14" to an integer in line 6, any information beyond the decimal point is lost forever. That’s why $undecided contains 3 after we change it back to a double in line 8. Finally, in line 10, we convert $undecided to a Boolean. Any number other than 0 becomes true when converted to a Boolean. When printing a Boolean in PHP, true is represented as 1, and false is represented as an empty string; so in line 11, $undecided is printed as 1.

When you access this script through your web browser, it produces the following output: is 3.14 a double? 1 is 3.14 a string? 1 is 3 an integer? 1 is 3 a double? 1 is 1 a boolean? 1 Note: You will not see a notice about an undefined variable like you did in the previous section, because the variable $undecided is defined and assigned a value at the beginning of this particular script.

Ch. 5: Changing Type with by Casting
The principal difference between using settype() to change the type of an existing variable and changing type by casting is the fact that casting produces a copy, leaving the original variable untouched. To change type through casting, you indicate the name of a data type, in parentheses, in front of the variable you are copying. For example, the following line creates a copy of the $originalvar variable, with a specific type (integer) and a new name $newvar. The $originalvar variable is still available, and is its original type; $newvar is a completely new variable: $newvar = (integer) $originalvar

Lab work: Listing 5.3 Casting a variable: 1: <?php 2: $undecided = 3.14; 3: $holder = (double) $undecided; 4: echo "is ".$holder." a double? ".is_double($holder)." "; // double 5: $holder = (string) $undecided; 6: echo "is ".$holder." a string? ".is_string($holder)." "; // string 7: $holder = (integer) $undecided; 8: echo "is ".$holder." an integer? ".is_integer($holder)." "; // integer 9: $holder = (double) $undecided; 10: echo "is ".$holder." a double? ".is_double($holder)." "; // double 11: $holder = (boolean) $undecided; 12: echo "is ".$holder." a boolean? ".is_bool($holder)." "; // boolean 13: echo "<hr/>"; 14: echo "original variable type of $undecided: "; 15: echo gettype($undecided); // double 16: ?>

Note: Listing 5.3 never actually changes the type of the $undecided variable, which remains a double throughout this script, as illustrated on line 15, where the gettype() function is used to determine the type of $undecided. Despite its usage here, do not use gettype() to test for a certain type because it can be slow and is likely to be deprecated in future versions. Use the is_* family of functions to test type in production, as shown in Listing 5.1. This usage is simply for illustrative purposes.

We can change the contents of a variable from one type to another, using either settype() or a cast, we should consider why this might be useful. It is certainly not a procedure that you will use often because PHP will automatically cast for you when the context requires it. However, an automatic cast is temporary, and you might wish to make a variable persistently hold a particular data type.

Numbers that a user types into an HTML form will be made available to your script as a string. If you try to add two strings containing numbers, PHP will helpfully convert the strings into numbers while the addition is taking place. So “30cm” + “40cm” will give the integer 70. In casting the strings, PHP will ignore the non-numeric characters.

Imagine that the user has been asked to submit a number. We can simulate this by declaring a variable and assigning to it: $test = “30cm”; We can make sure that the user input is clean by casting it to an integer: $newtest = (integer)$test; echo “Your imaginary box has a width of $test centimeters”;

Ch. 5: Why Test Type? There are often circumstances in programming in which data is passed to you from another source. Functions can accept information from calling code in the form of arguments. For the function to work with the data it is given, it is often a good idea to first verify that it has been given values of the correct data type. A function that is expecting a resource, for example, will not work well when passed a string.

Ch. 5: Operators and Expressions
An operator is a symbol or series of symbols that, when used in conjunction with values, performs an action and usually produces a new value. An operand is a value used in conjunction with an operator. There are usually two operands to one operator. An expression is any combination of functions, values, and operators that resolve to a value. As a rule of thumb, if you can use it as if it were a value, it is an expression.

Ch. 5: The Assignment Operator
The assignment operator each time we have initialized a variable; it consists of the single character =. The assignment operator takes the value of its right-hand operand and assigns it to its left-hand operand: $name = “matt”; The variable $name now contains the string “matt”. Interestingly, this construct is an expression.

Ch. 5: The Arithmetic Operators
The arithmetic operator perform arithmetic operations. Below are examples of arithmetic operators:

Ch. 5: The Concatenation Operator
The concatenation operator is represented by a single period (.). Treating both operands as strings, this operator appends the right-hand operand to the left. So “hello”.” world” returns “hello world” Regardless of the data types of the operands, they are treated as strings, and the result is always a string.

Ch. 5: Combined Assignment Operators
Each arithmetic operator, as well as the concatenation operator, also has a corresponding combination assignment operator. Here are examples:

Ch. 5: Automatically Incrementing and Decrementing an Integer Variable
When coding in PHP, it is necessary to increment or decrement an integer variable. You will usually need to do this when you are counting the iterations of a loop . You have already learned two ways of doing this. We can increment the integer contained by $x with the addition operator $x = $x + 1; // $x is incremented or with a combined assignment operator $x += 1; // $x is incremented

In both cases, the resultant integer is assigned to $x. Because expressions of this kind are so common, PHP provides some special operators that allow you to add or subtract the integer constant 1 from an integer variable, assigning the result to the variable itself. These are known as the post-increment and post-decrement operators. Post-increment operator consists of two plus symbols appended to a variable name: $x++; // $x is incremented

This expression increments the variable $x by one. Using two minus symbols in the same way decrements the variable: $x--; // $x is decremented If you use the post-increment or post-decrement operators in conjunction with a conditional operator, the operand will only be modified after the test has been completed: $x = 3; $y = $x++ + 3;

In some circumstances, you might want to increment or decrement a variable in a test expression before the test is carried out. PHP provides the pre-increment and pre-decrement operators for this purpose. These operators behave in exactly the same way as the post-increment and post-decrement operators, but they are written with the plus or minus symbols preceding the variable: ++$x; // $x is incremented --$x; // $x is decremented

If these operators are used as part of a test expression, the incrementing occurs before the test is carried out. $x = 3; ++$x < 4; // false In the previous fragment, $x is incremented before it is tested against 4. The test expression returns false because 4 is not smaller than 4

Ch. 5: Comparison Operators
Comparison operators perform tests on their operands. They return the Boolean value true if the test is successful, or false otherwise. This type of expression is useful in control structures, such as if and while statements. To test whether the value contained in $x is smaller than 5, you can use the less-than operator: $x < 5. If $x contains the value 3, this expression has the value true. If $x contains 7, the expression resolves to false

Ch. 5: Comparison Operators

Ch. 5: More Complex Test Expressions with Logical Operations
Logical operators test combinations of Booleans. For example, the or operator, which is indicated by two pipe characters (||) or simply the word or, returns true if either the left or the right operand is true: true || false This expression returns true

The and operator, which is indicated by two ampersand characters (&&) or simply the word and, returns true only if both the left and right operands are true: true && false This expression returns false. It’s unlikely that you will use a logical operator to test Boolean constants, as it makes more sense to test two or more expressions that resolve to a Boolean. For example ( $x > 2 ) && ( $x < 15 ) returns true if $x contains a value that is greater than 2 and smaller than 15

List of logical operators:

Ch. 5: Operator Precedence
The precedence of the operators in a complex expression, it is a good idea to use parentheses to make your code (those with highest precedence are listed first): ++, --, (cast) /, *, % +, - <, <=, =>, > ==, ===, != && || =, +=, -=, /=, *=, %=, .= and xor or

Ch. 5: Constants Variables offer a flexible way of storing data. You can change their values and the type of data they store at any time. You must use PHP’s built-in define() function to create a constant, which subsequently cannot be changed. To use the define() function, you must place the name of the constant and the value you want to give it within parentheses, separated by a comma: define(“CONSTANT_NAME”, 42);

Ch. 5: Predefined Constants
PHP automatically provides some built-in constants for you. For example, the constant __FILE__ returns the name of the file that the PHP engine is currently reading. The constant __LINE__ returns the line number of the file. These constants are useful for generating error messages. You can also find out which version of PHP is interpreting the script with the PHP_VERSION constant. This can be useful if you need version information included in script output when sending a bug report.

Flow Control Functions in PHP
Chapter 6 Flow Control Functions in PHP

Ch. 6: Outline In this chapter, we learn how to
implement decision-making and control within code, using, If and Switch statements. implement repetition, using for and while loops.

Ch. 6: Switching Flow Syntax of the if-statement is as follows:
if (expression) { // some code (one or more statements). } Expression is true  execute code Expression is false  skip the code

Ch. 6: Using the else clause with if
Control requires sometimes to take an action independent of whether a condition holds or not. Here, we use the if-else statement. Syntax of the if-else statement if (expression) { // first block (one or more statements) } else{ // second block (one or more statements)

Ch. 6: Using the else clause with if
Listing 6.2 1: <?php 2: $mood = "sad"; 3: if ($mood == "happy") { 4: echo "Hooray! I'm in a good mood!"; 5: } else { 6: echo "I'm in a $mood mood."; 7: } 8: ?>

Ch. 6: Using elseif clause with if
Sometimes, on needs to multiple expressions before taking a default action. Here, we use if..elseif..else statement. Syntax of the if-elseif-else statement if (first expression) { // first block (one or more statements) } elseif(second expression){ // second block (one or more statements) else{ // third block (one or more statements)

Ch. 6: Using elseif clause with if
Listing 6.3: 1: <?php 2: $mood = "sad"; 3: if ($mood == "happy") { 4: echo "Hooray! I'm in a good mood!"; 5: } elseif ($mood == "sad") { 6: echo "Awww. Don't be down!"; 7: } else { 8: echo "I'm neither happy nor sad, but $mood."; 9: } 10: ?>

Ch. 6: The Switch Statement
The switch statement is another way to make-decision during the flow of code. However, it differs from the if-statement in that it evaluates only one expression. Rather than being evaluated true or false as expressions in if-statement, the result of evaluating a switch expression is tested against any number of values.

Ch. 6: The Switch Statement
switch (expression) { case result1: // execute this if expression results in result1 break; case result2: // execute this if expression results in result2 default: // execute this if no break statement // has been encountered hitherto }

Ch. 6: Using the “?” Operator
Similar to the if-statement, we can make-decisions using the ternary operator “?”. However, it returns a value that corresponds to one of two expressions, separated by a colon “:” The expression used to generate the returned value depends on the result of a test expression. (expression) ? returned_if_expression_is_true : returned_if_expression_is_false;

Ch. 6: Using the “?” Operator
Listing 6.5 1: <?php 2: $mood = "sad"; 3: $text = ($mood == "happy") ? "I am in a good mood!" : "I am in a $mood mood."; 4: echo "$text"; 5: ?>

Ch. 6: Loops (While) The structure of a while loop resembles that of a basic if-statement. while (expression) { // do something };

Ch. 6: Loops (While) Listing 6.6
1: <?php 2: $counter = 1; 3: while ($counter <= 12) { 4: echo $counter." times 2 is ".($counter * 2)." "; 5: $counter++; 6: } 7: ?>

Ch. 6: Loops (do..while statement)
The do..while statement looks a little like a while statement turned on its head. It differs from the while statement in that it invokes an execution of the code before evaluating the expression. do { // code to be executed } while (expression);

Ch. 6: Loops (For) The for statement provides a compacter way for looping. for (initialization expression; test expression; modification expression) { // code to be executed }

Ch. 6: Breaking Out of Loops
Both while and for statements incorporate a built-in test expression for ending a loop. However, with the break statement we can break out of a loop on the basis of additional tests. This secures against errors.

Ch. 6: Breaking out of Loops
Listing 6.9 Dividing 4000 by 10 incremental numbers 1: <?php 2: for ($counter=1; $counter <= 10; $counter++) { 3: $temp = 4000/$counter; 4: echo "4000 divided by ".$counter." is...".$temp." "; 5: } 6: ?>

Ch. 6: Breaking out of Loops
Listing 6.10 1: <?php 2: $counter = -4; 3: for (; $counter <= 10; $counter++) { 4: if ($counter == 0) { 5: break; 6: } else { 7: $temp = 4000/$counter; 8: echo "4000 divided by ".$counter." is...".$temp." "; 9: } 10: } 11 ?>

Ch. 6: Skipping an Iteration with Continue
Listing 6.11 Skipping an iteration with the continue statement 1: <?php 2: $counter = -4; 3: for (; $counter <= 10; $counter++) { 4: if ($counter == 0) { 5: continue; 6: } 7: $temp = 4000/$counter; 8: echo "4000 divided by ".$counter." is...".$temp." "; 9: } 10: ?>

Ch. 6: Nesting Loops Useful, e.g. when working with dynamically created HTML tables. Listing 6.12 Nesting two “for” loops 1: <?php 2: echo "<table style=\"border: 1px solid #000;\"> \n"; 3: for ($y=1; $y<=12; $y++) { 4: echo "<tr> \n"; 5: for ($x=1; $x<=12; $x++) { 6: echo "<td style=\"border: 1px solid #000; width: 25px; 7: text-align:center;\">"; 8: echo ($x * $y); 9: echo "</td> \n"; 10: } 11: echo "</tr> \n"; 12: } 13: echo "</table>"; 14: ?>

Ch. 6: Nesting Loops

End of Session 1

M275 – Web Development Using PHP and MySQL

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