const FOO = 1;What are the main differences between those two?When và why should you use one and when use the other?
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As of PHP 5.3 there are two ways lớn define constants: Either using the const từ khoá 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, whereas define defines them at run time. This causes most of const"s disadvantages. Some disadvantages of const are:
const cannot be used to conditionally define constants. To define a global constant, it has to lớn be used in the outermost scope:
if (...) const FOO = "BAR"; // Invalid // but if (...) define("FOO", "BAR"); // Valid Why would you want to vị that anyway? 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 like true, false, null, __FILE__), whereas define() takes any expression. Since PHP 5.6 constant expressions are allowed in const as well:
const BIT_5 = 1 const takes a plain constant name, whereas define() accepts any expression as name. This allows to bởi things lượt thích this:
for ($i = 0; $i consts are always case sensitive, whereas define() allows you to lớn define case insensitive constants by passing true as the third argument (Note: defining case-insensitive constants is deprecated as of PHP 7.3.0 và removed since PHP 8.0.0):
define("FOO", "BAR", true); echo FOO; // BAR echo foo; // BARSo, that was the bad side of things. Now let"s look at the reason why I personally always use const unless one of the above situations occurs:
const simply reads nicer. It"s a language construct instead of a function & also is consistent with how you define constants in classes.
const, being a language construct, can be statically analysed by automated tooling.
const defines a constant in the current namespace, while define() has khổng lồ be passed the full namespace name:
namespace ABC; // to define the constant ABCFOO: const FOO = "BAR"; define("ABCFOO", "BAR");Since PHP 5.6 const constants can also be arrays, while define() does not tư vấn arrays yet. However, arrays will be supported for both cases in PHP 7.
const FOO = <1, 2, 3>; // Valid in PHP 5.6 define("FOO", <1, 2, 3>); // Invalid in PHP 5.6 & valid in PHP 7.0Finally, lưu ý that const can also be used within a class or interface khổng lồ define a class constant or interface constant. Define cannot be used for this purpose:
class Foo const BAR = 2; // Valid// Butclass Baz define("QUX", 2); // InvalidSummary
Unless you need any type of conditional or expressional definition, use consts instead of define()s - simply for the sake of readability!