example ,Conditional functions on PHP

$makefoo = true;

/* We can't call foo() from here
since it doesn't exist yet,
but we can call bar() */

bar();

if ($makefoo) {
function foo()
{
echo "I don't exist until program execution reaches me.\n";
}
}

/* Now we can safely call foo()
since $makefoo evaluated to true */

if ($makefoo) foo();

function bar()
{
echo "I exist immediately upon program start.\n";
}