Question

C++ preprocessor #define is totally different.

Is the PHP define() any different than just creating a var?

define("SETTING", 0);  
$something = SETTING;

vs

$setting = 0;  
$something = $setting;

Answer 1

‘define’ operation itself is rather slow – confirmed by xdebug profiler.

Here is benchmarks from http://t3.dotgnu.info/blog/php/my-first-php-extension.html:

• pure ‘define’
  380.785 fetches/sec
  14.2647 mean msecs/first-response

• constants defined with ‘hidef’ extension
  930.783 fetches/sec
  6.30279 mean msecs/first-response

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broken link update

The blog post referenced above has left the internet. It can still be viewed here via Wayback Machine. Here is another similar article.

The libraries the author references can be found here (apc_define_constants) and here (hidef extension).