it is good manner always declare function/class before using it.

Top level declaration are available after compiling, but conditional declaration
can only be available after executed. (since PHP itself didn't do optimization, 
i`f(true) {doSomething();}` is not equal to `doSomething();` exactly )

the same rules for class declarations.

This could be classified as a documentation problem.

dup to #17055

I accept that this is a duplicate of #17055.

And since derick said, in response to #17055, "I don't see any fast 
implementation of this.", 10 years ago (May 2002), I'm inclined to agree that 
this will have to be dealt with as a documentation problem.

However, in my particular scenario the original response to the problem... "The 
only way to prevent this is to move out the function declaration out of the 
branch and in the main scope of the script." is not acceptable since the whole 
point of the encapsulating code was to prevent the functions from being multiply 
defined.

So I believe the documentation should provide a clear explanation of the parsing 
and interpretation processes involved.

hmm, it's not the only way,  since you can get it works well by declare it 
before reference to it:
<?php
if ( true ) {
  function a() {
    echo "a" . PHP_EOL;
  }
  a();
}

laruence... agreed. That's how I solved it, as documented in my original report.
I was just commenting on the original response in #17055.
It was the unexpected behaviour that threw me more than anything.

@herb: You are using a conditional function definitions, in which case PHP *can't* know which function will be used, at least not in the general case. Function hoisting only works with "top-level" function declarations.

This is also documented on http://php.net/manual/en/functions.user-defined.php. See in particular those two lines:

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

> When a function is defined in a conditional manner such as the two examples shown. Its definition must be processed *prior* to being called.

A very simple solution to your particular problem (which at the same time will yield cleaner code) is to abort early if the constant is defined:

<?php
if (defined('CONSTANT')) {
    return;
}
// the rest of the file here

@nicic thanks for the link to the documentation. 
I hadn't realised I could use return... having previously failed to get a 
working solution with exit :-)  
I'll most likely change my code to something like this.
<?php
 if ( defined( 'CONSTANT' )) return;
 define( 'CONSTANT', true );
 a(); 
 function a() {
   echo "a" . PHP_EOL;
 } 

Then I don't have to worry about the final } at the end of the file
So should #17055 be marked as Closed now, rather than Suspended?