Cannot redeclare PHP Origin: Application & API Last updated 12 Oct 2026

Fatal error: Cannot redeclare function foo() (previously declared in …)

The same function (or class) was defined twice — usually a file included more than once.

Exact message you may see Cannot redeclareCannot declare classpreviously declared in
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Seen on: PHP

Meaning

PHP functions are global and can only be declared once per request. Using include/require instead of include_once/require_once, or two plugins defining the same helper, triggers this fatal error.

Common causes

  • File included twice with include/require
  • Two libraries/plugins define the same global function
  • Function defined inside a loop or another function that runs twice

⚡ Quick fix

  1. Use require_once
  2. Wrap helpers in if (!function_exists('foo')) { … }
  3. Move helpers into a namespace or class

Detailed fix by platform

WordPress

  1. Prefix plugin/theme functions with a unique name or wrap in function_exists checks

How to diagnose

  1. Locations — Both file paths are in the message
  2. Includes — Is the file included twice?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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