What is the difference between define and const in PHP?

In PHP, define and const are both used to define constants, which are values that cannot be changed once they are set. There are some differences between the two:

1. define is a function, while const is a language construct. This means that define can be called like a function (e.g., define('FOO', 'bar')), while const is used like a keyword (e.g., const FOO = 'bar').
2. define can define constants at runtime, while const can only define constants at compile time. This means that define can be used to define constants based on runtime conditions, while const can only be used to define constants with fixed values.
3. define constants are case-insensitive by default, while const constants are case-sensitive. This means that define('FOO', 'bar') and define('foo', 'bar') define the same constant, while const FOO = 'bar' and const foo = 'bar' define two different constants.

Here’s an example of how you can use define and const to define constants:

define('FOO', 'bar');
echo FOO;  // Outputs: bar
echo foo;  // Outputs: bar

const BAR = 'baz';
echo BAR;  // Outputs: baz
echo bar;  // Outputs: bar

In general, const is the recommended way to define constants in PHP, as it is faster and more flexible than define. However, define may be useful in certain situations where you need to define constants at runtime based on dynamic values.