PHP has its own list of reserved keywords which have special meaning. They may not be used as names for classes, functions, or constants.

Some of these you may already be familiar with such as “for”, “if”, and “class”. Others may be new to you such as “goto” and “namespace” (both of which were introduced in PHP 5.3).

New reserved words may be introduced with any new PHP release, though not too frequently and they are always carefully selected. You can see the list of reserved keywords here.