Creating Anonymous Functions

It is possible to create functions on the fly during script execution. Because such functions are not themselves given a name, but are stored in variables or passed to other functions, they are known as anonymous functions. PHP provides the create_function() function for creating anonymous functions. create_function() requires two string arguments. The first argument should contain a comma-delimited list of argument variables, exactly the same as the argument variables you would include in a standard function declaration. The second argument should contain the function body.

Listing 6.15 creates a simple anonymous function to add two numbers together.

Listing 6.15 A Simple Anonymous Function

  1: <html>
  2: <head>
  3: <title>Listing 6.15</title>
  4: </head>
  5: <body>
  6: <?php
  7: $my_anon = create_function( '$a, $b', 'return $a+$b;' );
  8: print $my_anon( 3, 9 );
  9: // prints 12
 10: ?>
 11: </body>
 12: </html>

As of this writing, the use of anonymous functions will cause a segmentation fault when running the Zend Optimizer.

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

Note that we use single quotes when passing arguments to create_function(). That saves us from having to escape the variable names within the arguments. We could have used double quotes, but the function call would have been a little more involved:

$my_anon = create_function("\$a, \$b", "return \$a+\$b;");

So what is the use of anonymous functions? In practical terms, you will probably only use them when you need to pass callback functions to built-in functions. A callback function is generally written by the user and is designed to be invoked (usually repeatedly) by the function to which it is passed.

The second argument to create_function() is the function body. Don't forget to end the last statement in this string with a semicolon. The interpreter will complain and your anonymous function will not be executed if you omit it.