How efficient is define in PHP?

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

---

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).

Answer 2

In general, the idea of a constant is to be constant, (Sounds funny, right? ;)) inside your program. Which means that the compiler (interpreter) will replace "FOOBAR" with FOOBAR's value throughout your entire script.

So much for the theory and the advantages - if you compile. Now PHP is pretty dynamic and in most cases you will not notice a different because the PHP script is compiled with each run. Afai-can-tell you should not see a notable difference in speed between constants and variables unless you use a byte-code cache such as APC, Zend Optimizer or eAccelerator. Then it can make sense.

All other advantages/disadvantages of constants have been already noted here and can be found in the PHP manual.