PHP Function Call Placement

Question

Consider this snippet:

function f() {
    return 'hi';
}

echo f();

Vs this snippet:

echo f();

function f() {
    return 'hi';
}

When I run the scripts, they both produce the same results. That's great.

But my question is (and I could not find a good answer or reference in any of my searches), is it okay to call the function before it is defined (ie, from a sequential parsing of the script file perspective)?

I don't want to run into any issues or deprecation down the road if I leave the function call ahead of the function definition block in my script file.

Answer 1

From the Manual:

Functions need not be defined before they are referenced, except when a function is conditionally defined as shown in the two examples below.

The possibility to call (reference) a function before it is defined is a PHP intentional feature and I don't think you need to worry about it becoming deprecated.

As an observation, if you can choose from declaring the function before or after, it would be common sense to declare it before it's used.

Note: The following code will give a fatal error because the function will only be defined at run rime.

<?php
echo helloWorld();
if(1){
    function helloWorld() {
        return 'hello world';
    }
}
?>

Answer 2

compiler steps are like so:

• Converts a sequence of characters into tokens
• Analyses the tokens to determine there Grammatical structure.
• Generates byte code depending on the outcome of the analyses

So the easiest way to understand this is just because the script is not multi threaded does not mean its processed in one in line execution.

PHP Reads your entire source code into tokens before its executed, there for it has control over the order of tokens should be executed first.

Take this example

while(true)
{
    print '*';
}

Each line is a sequence of characters, so PHP Would interpret this as

if          #T_IF
            #T_WHITESPACE
(
            #T_WHITESPACE
true        #T_STRING
            #T_WHITESPACE
)
            #T_WHITESPACE
{
            #T_WHITESPACE
print       #T_PRINT
            #T_WHITESPACE
'*';        #T_CONSTANT_ESCAPED_STRING
            #T_WHITESPACE
}

but just because its been read does not mean its been executed.

So that functions are at the top of the list, this way you can execute them because there already within the systems memory.

I believe that the reason for this is that PHP's native library such as PFO,mysql_connect functions and classes are loaded first, and they move all user defined scopes to be loaded after there native implementations.

there loaded at the beginning of execution.