Why in PHP is it possible to access functions before the line they were declared?

Why in PHP is it possible to call a function before the line where it was declared?

echo ah(); // 'Ah!'

function ah() {
     return 'Ah!';
}

Notice that I called ah() first, then declared. Even so, she was executed. But the function theoretically would not exist before this line? So how was she called?

How does this happen internally? I thought the script would run sequentially, but calling a function that is declared after that gives me another understanding.

This seems to work only when the statement is made in the same script.

When a script is added after the function call, this does not happen, even if within the include function call exists.

index.php

echo ah();

include 'ah.php';

a.php

function ah() { return 'Ah!'; }

This would generate:

  Uncaught Error: Call to undefined function ah () in index.php

Why?

• What is the difference between the function declared after the call that is in the same script for the function that was called before the statement coming from a include ?

• In the first case, does PHP only parse the code once to see if the function has been declared? How did this "magic" come about?

First example on IDEONE