Defining a function inside a conditional

Snippet 1 works. Snippet 2 doesn't. Why?

1.

foo();

function foo()
{
    // do soemething
}

2.

foo();

if(!function_exists("foo"))
{
    function foo()
    {
        // do soemething
    }
}

See http://www.php.net/manual/en/functions.user-defined.php:

Functions need not be defined before they are referenced, except when a function is conditionally defined [...] Its definition must be processed prior to being called.

You're trying to execute foo() before testing to see whether it's defined or not (and subsequently defining it)

if(!function_exists("foo")) 
{ 
    function foo() 
    { 
        // do soemething 
    } 
} 

foo();