PHP Variables

What is a variable? A variable is something that can be changed – a thing that can take on any value from a possible set of values. For example, the number of hours someone works in a given week is a variable. In math, a symbol is used in place of the variable to express a relationship between some quantity and the variable For example, pay is one’s pay rate times the number of hours worked Pay = PayRate * NumberHours

What is a variable? (Cont.) So a variable is something that stands in for a quantity that might vary. In programming, a variable can be thought of a referring to a memory location or set of memory locations. The value of the variable at any given time is then the contents of that memory location As we said a variable can take on values from a set of possible values, the set of values is determined in part by the variable’s type.

Types Unlike many other programming languages, PHP variables do not have to be “declared.” In such a case, a variable’s types tends to come from its context. Standard types includes numbers (both integers and so-called floating point numbers that can have fraction parts. Another standard type is a string,which can used for names of things or just about any text.

PHP variable rules In PHP a variable starts with a dollar sign. After that must come a letter or an underscore (no numbers). After that can come letters or numbers. There can be no spaces in the name.

Assignment A variable is said to be assigned a value in an assignment statement. E.g. $NumberHours = 45; $WorkerFirstName = “Pete”; The assignment statement should not be thought of as expressing mathematical equality rather it a set of instructions to evaluate the expression on the right hand side and give the variable on the left hand side the value that results. Thus $Counter = $Counter + 1; Makes sense as an assignment statement even though it makes no sense as mathematics.

Turning DateDisplay into a program with a variable.

Result of DateDisplay as variable.

Incorporating variables within strings. print "The date is $dateString."; The above code also shows how variables are used in conjunction with strings in PHP. Within the string (set of characters within quotes) that will appear literally (“as is”), the variable does not appear as is, rather its value is substituted into the string.

If you don’t want the variable value substituted in, then use single quotes.

Result of single quoted version of DateDisplay.

That variable doesn’t vary much. Because these are PHP variables, all of their variation takes place on the server side (before the PHP preprocessed page is sent to the client). PHP variables will ultimately give us interaction with the viewer of the page but always through a process in which information is sent back to the server.

The bells, bells, bells example. Before consider variables determined by interaction with a user, let us consider one example in which the variable does vary. In Edgar Allen Poe’s poem The Bells, the word “bells” appears over and over again. Let’s substitute in an image of bells in place of the word bells. So far substitution (a lot of it) but not variation.

The bells, bells, bells example. (Cont.) The first part of the poem talks about silver bells, the second about gold bells, and so on. So in this example we will vary the image and the font color for each of the different parts.

Assigning the variables Writing some HTML A chunk of the poem, with the word bells replaced with the variable $bells. Note that the quotation can be spread out over many lines. (That would ne a no-no in C.) Re-assigning the variables

Where we left off. More of the same.

Silver bells turning into Golden bells as a result of our variable being reassigned.

Constants If you really want a quantity that is going to take on a value and never change then what you want is called a constant. Constants do not start with dollar signs. By convention, constant names are in all capital letters.

Pi page (code)

Pi page (in browser)

PHP’s Global Constants The <pre> tag tells HTML to respect the white space of the text between the tags. The function print_r is for printing arrays – GLOBALS is not one constant but a set of constants

Some of these constants tell one information about the PHP server – in this case the alpha.