The difference between the use of const and define in PHP

1. Const is used for class member variable definition, once defined and cannot change its value. Define defines global constants that can be accessed everywhere.

2. Define cannot be defined in a class and const can.

3. Const cannot define constants in conditional statements

if (...) {
Const FOO = ' BAR '; Invalid
}

But

if (...) {
Define (' FOO ', ' BAR '); Valid
}

4, const uses a common constant name, define can use the expression as the name.

Const FOO = ' BAR ';

for ($i = 0; $i < + + + $i) {
Define (' Bit_ '. $i, 1 << $i);
}

5, const can only accept static scalar, and define can use any expression.

Const BIT_5 = 1 << 5; Invalid

But

Define (' Bit_5 ', 1 << 5); Valid

6, const is always case sensitive, however define () can use the third parameter to define a case insensitive constant

Define (' FOO ', ' BAR ', true); Www.2cto.com
Echo FOO; BAR
echo foo; BAR
Summarize:
The use of const is simple and easy to read, it is itself a language structure, and define is a method, with a const definition at compile time much faster than define.

The difference between the use of const and define in PHP