除了使用include_once / require_once之外，什么是阻止PHP类重新定义的最有效方法？ [关闭]

Or in other words, is there a PHP equivalent of the C++ directives #ifndef, #define which are used to accomplish this?

I figure this will work:

<?php

if(!class_exists("MyClass")):

class MyClass
{
    ...
}

endif;

?>

This avoids the overhead that comes with require_once/include_once functions.

Well if you really like conditional including to reduce "PHP symbol table" you can code like C preprocessor directives like this:

IF ( !defined( 'MY_CONSTANT' ):
  define( 'MY_CONSTANT', 'PHP C like defined' );
ENDIF;

IF ( !defined( 'header_php' ):
  define( 'header_php', 'PHP "header" defined' );
  include( 'header.php' );
ENDIF;

// you can do this for class, functions, variables etc

But on the end will have additional cosntants symbols defined for nothing.