php的常量定义:const VS define

As of PHP 5.3 there are two ways to define constants: Either using the const keyword or using the define() function:

const FOO = 'BAR'; define('FOO', 'BAR'); 

The fundamental difference between those two ways is that const defines constants at compile time, whereasdefine defines them at run time. What does that mean for you?

• const cannot be used to conditionally define constants. It has to be used in the outermost scope:

  if (...) {     const FOO = 'BAR';    // invalid } // but if (...) {     define('FOO', 'BAR'); // valid } 

  Why would you want to do that anyways? One common application is to check whether the constant is already defined:

  if (!defined('FOO')) {     define('FOO', 'BAR'); } 

• const accepts a static scalar (number, string or other constant liketrue, false, null, __FILE__), whereasdefine() takes any expression:

  const BIT_5 = 1 << 5;    // invalid define('BIT_5', 1 << 5); // valid 

• const takes a plain constant name, whereas define() accepts any expression as name. This allows to do things like this:

  for ($i = 0; $i < 32; ++$i) {     define('BIT_' . $i, 1 << $i); } 

• consts are always case sensitive, whereas define() allows you to define case insensitive constants by passingtrue as the third argument:

  define('FOO', 'BAR', true); echo FOO; // BAR echo foo; // BAR 

So, that was the bad side of things. Now let's look at the reason why I personally always useconst unless one of the above situations occurs:

• const simply reads nicer. It's a language construct instead of a function and also is consistent with how you define constants in classes.

• As consts are language constructs and defined at compile time they are a bit faster thandefine()s.

  It is well known that PHP define()s are slow when using a large number of constants. People have even invented things likeapc_load_constants() andhidef to get around this.

  consts make the definition of constants approximately twice as fast (on development machines with XDebug turned on even more). Lookup time on the other hand does not change (as both constant types share the same lookup table):Demo.

Summary

Unless you need any type of conditional or expressional definition, use consts instead of define()s - simply for the sake of readability!