Silimiar to C++, PHP use define keyword to declare a constant.

<?PHP
	define("PI",3.1415926);
	define("OOP", "Object Oriented Programming");
?>

Function constant() returns the value of a constant.

<?PHP
	echo PI; //Print 3.1415926
	echo "PI"; //Print PI
	echo constant("PI");  //Print 3.1415926
?>

PHP built in constants list:

PHP ERROR constants:
E_ERROR, E_WARNING, E_PARSE, E_NOTICE, E_CORE_ERROR, E_CORE_WARNING, E_COMPILE_ERROR, E_COMPILE_WARNING, E_USER_ERROR, E_USER_WARNING, E_USER_NOTICE, E_DEPRECATED, E_USER_DEPRECATED, E_ALL, E_STRICT.

Adjust error reporting using the ERROR constants:

<?PHP
	error_reporting(0); //no error reporting
	error_reporting(E_ALL); //report all errors
	error_reporting(E_ALL ^ E_WARNING); //report all errors except E_WARNING
	error_reporting(E_ERROR | E_WARNING); //report E_ERROR and E_WARNING
?>