This can be used to conditionally define a user function. In this sense, it can act as a sort of inline include_once().
For example, suppose you have a function A that calls function B. B is only used inside function A and is never called from anywhere else in the script. It's logical (and perfectly legal in PHP) to define B inside of A's definition, like so:
<?php
function A($inputArray)
{
if (!function_exists('B'))
{
function B($item)
{
return $result;
}
}
foreach ($inputArray as $nextItem) $outputArray[] = B($nextItem);
return $outputArray;
}
?>
Without the function_exists test, you would get a fatal error the second time you called A, as PHP would think you were trying to redefine B (not legal in PHP). The placement of the test is also important. Since the if block is executed sequentially, like any other block of code, it must come before any call to the function defined within.
