Create_function enables the ability to change the scope of functions.  You might have a class where it needs to define a GLOBAL function.  This is possible, like:

<?php
        class blah {
                function blah() {
                        $z=create_function('$arg1string','return "function-z-".$arg1string;');
                        $GLOBALS['z']=$z;
                }
        }
        $blah_object=new blah;

        $result=$GLOBALS['z']('Argument 1 String');
        echo $result;
?>

Making a function escape it's defined scope can be useful in many situations.