PHP Fatal error: Cannot redeclare ...

The “Cannot redeclare” fatal error occurs when PHP encounters a second definition of a function or class that has already been defined. This is a hard error that stops execution immediately.

What It Means

PHP allows each function and class to be defined only once in a given request. If you attempt to define the same function twice — even with identical code — PHP throws a fatal error. The same applies to classes, interfaces, and traits.

Why It Happens

• File included multiple times — a file containing a function definition is included or required twice.
• Function defined conditionally but executed twice — a function inside a loop or conditional branch is evaluated more than once.
• Plugin or framework conflict — two extensions define the same function name.
• Manual include after autoloader — Composer’s autoloader loads a class, then you try to require the same file manually.
• Copy-pasted code — the same function definition exists in two different files that both get included.

How to Fix It

1. Use require_once instead of require or include

// ❌ Can cause "cannot redeclare" if included twice
require 'helpers.php';
require 'helpers.php';

// ✅ Prevents double inclusion
require_once 'helpers.php';

2. Check for existing definition before declaring

if (!function_exists('formatDate')) {
    function formatDate($timestamp) {
        return date('Y-m-d', $timestamp);
    }
}

3. Find duplicate includes with debug backtrace

// Temporarily add this to find where the function is declared
$declared = get_defined_functions()['user'];
if (in_array('yourFunctionName', $declared)) {
    $refl = new ReflectionFunction('yourFunctionName');
    echo "Already defined in: " . $refl->getFileName();
}

4. Restructure your autoloading

If using Composer, let the autoloader handle includes:

{
    "autoload": {
        "files": ["src/helpers.php"],
        "classmap": ["src/"]
    }
}

Run composer dump-autoload after updating.