Deep analysis of PHP const and define usage differences

Note: Define cannot be defined in a class, and the const must be defined in a class, and the const must be accessed by the class name:: Variable name

1. Const is used for class member variable definition once defined and cannot change its value. Define defines global constants that can be accessed anywhere.
2, define can not be defined in the class and Const.
3. Const cannot define constants in conditional statements

4, the const use a common constant name, define can use an expression as the name.
5, const can only accept static scalar, and define can take any expression.
6. Const is always case sensitive, but define () can define a case insensitive constant with a third argument
7. Using the const simple and easy to read, which is itself a language structure, and define is a method, with the const definition at compile time much faster than define.

Define is defined as constants, what if constants are defined in the class? Certainly cannot use define, but uses the const, the following example:

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<?php Outside of the class usually this defines constants Define ("PHP", "111cn.net"); Class MyClass { The value of the constant will always remain unchanged. You do not need to use the $ symbol when defining and using constants CONST CONSTANT = ' constant value '; function Showconstant () { Echo Self::constant. "<br>"; } } Echo Myclass::constant. "<br>"; $classname = "MyClass"; echo $classname:: Constant. "<br>"; After PHP 5.3.0 $class = new MyClass (); $class->showconstant (); echo $class:: Constant. " <br> "; After PHP 5.3.0 Print_r (Get_defined_constants ()); You can get all the defined constants with Get_defined_constants () ?>

Typically, define defines constants outside the class, const defines constants within the class, and const must be accessed through the class name:: Variable name. But php5.3 above the support class to define constants by const, see below, this is OK:

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<?php //@blog Const A = "abcdef"; echo A; ?>

The basics of constants are not mentioned here, except for the other differences between define and const (excerpt from the network):

1.const cannot define constants in a conditional statement, but define is OK, as follows:

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<?php if (1) { Const A = ' Java '; } echo A; Must be wrong ?>

2.const uses an ordinary constant name, define can use an expression as the name

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<?php Const FOO = ' PHP '; for ($i = 0; $i < + + $i) { Define (' Php_ '. $i, 1 << $i); } ?>

3.const can only accept static scalars, and define may take any expression.

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<?php Const PHP = 1 << 5; Error Define (' PHP ', 1 << 5); That's right ?>

4.const itself is a language structure. and define is a function. So it's much faster to use the const.

Two sync: Both are not assignable.