Fix: Undefined constant error PHP fix

The ‘Undefined constant’ error occurs when PHP encounters a constant that hasn’t been defined, often due to typos, missing definitions, or scope issues.

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How the Error Happens

❌ Error Scenario:

<?php
// ❌ This causes the error
echo API_URL; // Fatal error: Uncaught Error: Undefined constant "API_URL"
?>

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✅ Quick Fix - Define and Use Constants Properly

Solution 1: Define Constants Before Use

<?php
// ✅ Define constant before using it
define('API_URL', 'https://api.example.com');

// ✅ Now you can use the constant
echo API_URL;
?>

Solution 2: Use define() with Conditional Check

<?php
// ✅ Check if constant exists before defining
if (!defined('API_URL')) {
    define('API_URL', 'https://api.example.com');
}

echo API_URL;
?>

Solution 3: Use Class Constants

<?php
// ✅ Define constants in a class
class Config {
    const DB_HOST = 'localhost';
    const DB_NAME = 'myapp';
    const API_KEY = 'your-api-key';
}

// ✅ Access class constants
echo Config::DB_HOST;
echo Config::DB_NAME;
?>

Solution 4: Use PHP 8.1+ Enum Constants

<?php
// ✅ Use enums for related constants (PHP 8.1+)
enum Status: string {
    case PENDING = 'pending';
    case APPROVED = 'approved';
    case REJECTED = 'rejected';
}

// ✅ Use enum constants
echo Status::PENDING->value;
?>

Solution 5: Check Constant Definition

<?php
// ✅ Verify constant exists before using
if (defined('API_URL')) {
    echo API_URL;
} else {
    echo "Constant API_URL is not defined";
}

// ✅ Or use a default value
$apiUrl = defined('API_URL') ? API_URL : 'https://default-api.com';
echo $apiUrl;
?>