Calling Functions

Functions come in two flavors those built in to the language and those you define yourself. PHP has hundreds of built-in functions. The very first script in this book, which appears in Hour 3, "Installing and Configuring PHP," consists of a single function call:

 print "Hello Web!";

In this example, we call the print() function, passing it the string "Hello Web!". The function then goes about the business of writing the string. A function call consists of the function name (print in this case) followed by parentheses. If you want to pass information to the function, you place it between these parentheses. A piece of information passed to a function in this way is called an argument. Some functions require that more than one argument be passed to them. Arguments in such cases must be separated by commas:

 some_function( $an_argument, $another_argument);

print() is typical in that it returns a value. Most functions give you some information back when they've completed their task they usually at least tell whether their mission was successful. print() returns a Boolean.

The abs() function, for example, requires a signed numeric value and returns the absolute value of that number. Let's try it out in Listing 6.1.

print() is not a typical function in that it does not require parentheses in order to run successfully:

 print("Hello Web!");

and

 print "Hello Web!";

are equally valid. This is an exception. All other functions require parentheses, whether or not they accept arguments.

Listing 6.1 Calling the Built-in `abs()` Function

   1: <html>   2: <head>   3: <title>Listing 6.1</title>   4: </head>   5: <body>   6: <?php   7: $num = -321;   8: $newnum = abs( $num );   9: print $newnum;  10: // prints "321"  11: ?>  12: </body>  13: </html>

In this example, we assign the value -321 to a variable $num. We then pass that variable to the abs() function, which makes the necessary calculation and returns a new value. We assign this to the variable $newnum and print the result.

Put these lines into a text file called abs.php, and place this file in your Web server document root. When you access this script through your Web browser, it produces the following:

In fact, we could have dispensed with temporary variables altogether, passing our number straight to abs(), and directly printing the result:

 print( abs( -321 ) );

We used the temporary variables $num and $newnum, though, to make each step of the process as clear as possible. Sometimes you can make your code more readable by breaking it up into a greater number of simple expressions.

You can call user-defined functions in exactly the same way that we have been calling built-in functions.

Listing 6.1 Calling the Built-in abs() Function

Listing 6.1 Calling the Built-in `abs()` Function