1 PHP Introduction Chapter 1. Syntax and language constructs

PHP’s Type Strength PHP is a dynamically typed language (a.k.a. weakly typed language): ○ The type of a variable is not / cannot be (!) declared when you create a new variable ○ The type of a variable is automatically determined by the value assigned to it ○ The type of a variable can change throughout the program according to what is stored in it at any given time $totalqty = 0; // integer $totalamount = 0.00; // float $totalamount = ‘Hello’; // string 2

PHP’s Type Casting You can force a data type by type casting ○ Syntax: put target type in parentheses in front of variable/expression you want to cast (similar to Java): $totalqty = 0; $totalamount = (float) $totalqty; // $totalamount variable will be of type float and // cast variable doesn’t change types! Built-in functions are also available to test and set type (see later) 3

Constants  Use the define() function to create a constant define("CONSTANT_NAME", value);  Constant names: Do not begin with a dollar sign (!) It is common practice to use all uppercase letters for constant names Are case sensitive by default (this can be changed, but not recommended!)  The value you pass to the define() function can only be a scalar value: a string, integer, float, or boolean value define(“VOTING_AGE”, 18); echo “ The legal voting age is ”, VOTING_AGE, " ";  Note no $ sign precedes constant names in expressions!  A constant’s name cannot be used as variable names within the quotation signs surrounding a string!  A list of predefined vars and constants can be obtained calling phpinfo() 4

Variables’ Scope I. PHP superglobals: 5

Variables’ Scope - 6 basic scope rules I. Built-in superglobal or autoglobal variables are visible everywhere (=globally) within a script, both inside and outside functions.  PHP includes various autoglobals or superglobals, which are predefined global arrays.  Autoglobals contain client, server, and environment information that you can use in your scripts.  Autoglobals are associative arrays – arrays whose elements are referred to with an alphanumeric key instead of an index number. 6

PHP Operators  Arithmetic operators: Binary operators +, -, *, /, %  Usually applied to integers or doubles  String operands are converted to numbers: starting at the beginning of the string up to 1 st non-valid character; if no valid characters, the value of the string will be 0. Unary operators +, -  String operators:. – the string concatenation operator $s1 = “Bob’s ”; $s2 = “Auto Parts”; $result = $s1. $s2; // result now contains the string “Bob’s Auto Parts” 7

Operators  Crash_Course_Examples/operators.php Crash_Course_Examples/operators.php  ples/PHP_Crash_Course/operators_php.pdf ples/PHP_Crash_Course/operators_php.pdf

PHP Operators  Assignment operators: Basic assignment operator =  The value of an assignment expression lhs_operand = rhs_expression is the value that is assigned to lhs_operand  = associates from right to left  expressions like the following one are correct: $a = $b = 6 + $c = 9; // $c is now 9, $a and $b are 15 // can also be written as $a = $b = 6 + ($c = 9); Combined assignment operators: +=, -=, *=, /=, %=,.=  $a += 5;is equivalent to $a = $a + 5; 9

PHP Operators  Assignment operators: Pre- and Post-Increment and Decrement: ++, --  All 4 operators modify the operand and the value of the expression is the old value (post) or the new value (pre) of the operand (like in Java). Reference operator: &  $a = 5; $b = $a;  a copy of the value in $a is made and stored in $b, elsewhere in memory  $b = &$a;  $b is an alias for the same memory location which is referred to as $a = $a and $b are associated with / name the same memory location  $a = 7; //  both $a and $b are now 7  unset($b) breaks the link between name $b and the memory location storing 7 → $b is now undefined 10

PHP Operators  Comparison operators: ==equals ===identical (if operands are equal and of the same type) !=not equal <>not equal !==not identical < > <= >= string comp_op integer → string is converted to a number 0==‘0’ → true 0 === ‘0’ → false string comp_op string → compared as numbers if they are numerical strings… 11

PHP Operators  Logical operators (combine the results of logical conditions): !NOT &&AND ||OR andAND (but lower precedence than &&) orOR (but lower precedence than ||) xorEXCLUSIVE OR ($a xor $b is true when exactly one of $a and $b is true)  The execution operator ``(= a pair of backticks, not quotes!) PHP tries to execute the content between `` as a command at the server’s command line. The value of the expression is the output of that command. Example: $out = `dir`; // on a Win server, the list of files/directories in current directory = where the script is echo “ $out ”; 12

PHP Operators - Precedence & Associativity  Precedence = order in which operations in an expression are evaluated  Associativity = order in which operators of equal precedence are evaluated 13 $a=5; $b=4; ? The value of the following expression? $b + ++$a/2 + 3 == 10

Using Variable Functions  Testing and setting variable types: string gettype(mixed variable) Returns one of the strings: “bool”, “int”, “double”, “string”, “array”, “object”, “resource”, “NULL”, or “unknown type”. “Mixed” – is a pseudo-type / signifies that variables of many (or any) data types are accepted as arguments; or function is “oveloaded” bool settype(mixed variable, string type) Sets the type of “variable” to the new type passed as a second argument. “Type” can be “bool”, “int”, “double”, “string”, “array”, “object”, “resource”, “NULL”. $a = 56; echo gettype($a).’ ’;// displays int settype($a, ‘double’); echo gettype($a).’ ’;// displays double 14

Using Variable Functions  Specific type-testing functions, return true of false is_array(variable) is_double(variable)is_float(variable)is_real(variable) is_long(variable)is_int(variable)is_integer(variable) is_string(variable) is_bool(variable) is_object(variable) is_resource(variable) is_null(variable) is_scalar(variable) – checks whether variable is an integer, boolean, string or float is_numeric(variable) – checks whether variable is any kind of number or a numeric string is_callable(variable) – checks whether variable is the name of a valid function 15

Using Variable Functions  Testing variable status: bool isset(mixed variable1 [, mixed variable2 …] ) Returns true if all variables passed to the function exist and false otherwise void unset(mixed variable1 [, mixed variable2, …]) Gets rid of the variable(s) it is passed bool empty(mixed variable) Checks if variable exists and has a nonempty, nonzero value These functions are very useful for checking (server-side) if the user filled out the appropriate fields in the form; example: echo isset($tireqty); // always true for a textbox field echo empty($tireqty); // depends on user input, false if form field is empty 16

PHP Conditionals  if (condition) statement or code_block where code_block = { statement(s) }  if (condition) statement or code_block else statement or code_block  NOTE: when writing a cascading set of if-else statements “else if” can be also written “elseif”  Conditional operator: condition ? value_if_true : value_if_false  switch Same syntax as Java; control expression and case labels can evaluate to integer, string or float Must use break to end execution after a matching case 17

PHP Conditionals - Example  HTML form contains the following select list: I'm a regular customer TV advertising …  Equivalent if-else and switch statements for processing form data for the select form field: $find = $_POST["find"]; if ($find == "a") echo " Regular customer. "; elseif ($find == "b") echo " Customer referred by TV advertising. "; … switch($find) { case "a": echo " Regular customer. "; break; … } 18

PHP Conditionals - Example  Crash_Course_Examples/orderform_3.h tml Crash_Course_Examples/orderform_3.h tml  ples/PHP_Crash_Course/processorder_ 3_php.pdf ples/PHP_Crash_Course/processorder_ 3_php.pdf

PHP Loop Statements  while ( condition ) // same semantics as in Java statement or code_block  Example - generate a shipping cost table, if prices are proportional to the distance: Distance Cost <?php $distance = 50; while ($distance <= 250 ) { echo " \n $distance \n"; echo " ". $distance/10." \n \n"; $distance += 50; } ?> 20

PHP Loop Statements  Crash_Course_Examples/freight.php Crash_Course_Examples/freight.php  ples/PHP_Crash_Course/freight_php.pd f ples/PHP_Crash_Course/freight_php.pd f

PHP Loop Statements  do statement or code_block while ( condition ); // same semantics as in Java  for (initial-action; loop-continuation-condition; action-after-each-iteration) statement or code_block // same semantics as in Java  Interesting example – combine variable variables with a for loop to iterate through and process a series of form fields with repetitive names, such as name1, name2, name3 etc: for ($i=1; $i<=$numfields; $i++) { $temp = “name$i”; // $i is interpolated! echo $$temp. “ ”; } 22

Alternative Control Structure Syntax  control_structure (expression_or_condition) : statements; special_keyword;  The alternative syntax can be used for: statement special_keyword is: if endif switch endswitch while endwhile for endfor  if ($totalqty == 0) : echo " You didn’t order anything on the previous page! "; exit; endif; 23