Constant Redefinition

<?php

 const A = 1;
 const A  =1; // warning!

 class X {
     const B = 2;
     const B = 2; // fatal error
 }

It is possible to specify several identical global constant definition: PHP reports a warning, and ignores the second definitions.

On the other hand, a duplicate class constant definition is a fatal error, and fails the compilation phase.

The difference of behavior may be linked to the level of consistence that PHP can achieve: global constants needs an application wide check for name unicity, while class constants can be checked while compiling a class.

Yet, it might be worth looking into leveling both errors, to avoid hard to find bugs.

See Also

• Constants (PHP manual)

• Redefinition of a constant [Try me]

PHP Error Messages

• Cannot redefine class constant X::B

PHP Features

• constant

• definition

• class-constant

Last updated: 14 July 2026