In this post, we give you the difference between const and define in PHP.

1. Compare define() vs const:

As we know both define() and const are used to declare a constant in PHP script.

Let’s discuss the difference between these two.

	Define	Const
1.Syntax	define(‘SOL’, ‘SOLTUTS’);	const SOL= ‘SOLTUTS’;
2. Run	define() defines them at run time. define()s are slow when using a large number of constants.	const defines constants at compile time, they are a bit faster than define()s.
3. Conditional blocks	We can use the define() keyword to declare constant in conditional blocks: EX: if (/* some condition */) { define(‘SOL’, ‘SOLTUTS’); //valid }	We can’t use the const keyword to declare constant in conditional blocks: EX:  if (/* some condition */) { const SOL = 'SOLTUTS'; // CANNOT DO THIS! }
4. Expression	define() takes any expression. Ex: define(‘BIT_5’, 1 << 5); // always valid for ($i = 0; $i < 32; ++$i) { define(‘BIT_’ . $i, 1 << $i); } define(‘CONST’, 4 * 3); // always valid	const accepts a static scalar (number, string or other constant like true, false, null, __FILE__). Since PHP 5.6 constant expressions are allowed in const as well. Ex: const BIT_5 = 1 << 5; // valid since PHP 5.6, invalid previously. const FOO = 4 * 3; // valid since PHP 5.6, invalid previously.
5. Case sensitive	define() allows you to define case insensitive constants by passing true as the third argument. Ex: define(‘SOL’, ‘SOLTUTS’, true); echo SOL; // SOLTUTS echo sol; // SOLTUTS	consts are always case sensitive. Ex: const SOL= 2; const sol = 3; echo SOL; // 2 echo sol; // 3
6. Utilized within a class	define() can’t be utilized within a class or interface to declare a class constant or interface constant. Ex: class Soltuts{       define(‘SOL’, 2); // invalid       echo SOL; // invalid    }	const can also be utilized within a class or interface to declare a class constant or interface constant Ex: class Soltuts{       const SOL= 2; // valid       echo SOL; // valid    }
7. Arrays	define() does not support arrays yet. Arrays will be supported in PHP 7. Ex: define(‘FOO’, [1, 2, 3]); // invalid in PHP 5.6, valid in PHP 7.0	const constants can also be arrays. Ex: const SOL = [1, 2, 3]; // valid in PHP 5.6
8. Namespace	define() has to be passed the full namespace name: Ex: namespace A\B\C; // To define the constant A\B\C\SOL: define(‘A\B\C\SOL’, ‘SOLTUTS’);	const defines a constant in the current namespace. Ex: namespace A\B\C; // To define the constant A\B\C\SOL: const SOL= ‘SOLTUTS’;