Analysis of the difference between const and define in php

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

4. const uses an ordinary constant name, and define can use an expression as the name. For example:

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

5. const can only accept static scalars, while define can use any expression. For example:

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

6, const Always case-sensitive, however define() can define case-insensitive constants via the third argument. For example:

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

From the above example, we get, const is simple and easy to read. It is a language structure in itself, while define is a method. Defining with const is much faster than define at compile time. As for which one to use, it depends on your own needs.