PHP Fatal Error: Call to Undefined Function – Step-by-Step Fix

Fix PHP Fatal Error: Call to Undefined Function – Step-by-Step Guide

Are you seeing the error PHP Fatal Error: Call to Undefined Function in your script? Don’t worry! In this blog, we’ll break down the root cause and walk you through a working fix that you can copy, paste, and run locally.

🧠 What Does This Error Mean?

This error means PHP tried to call a function that hasn’t been defined or isn’t available in your environment. This often happens when:

• You misspelled the function name
• You forgot to include the necessary PHP extension or file
• The function is from a disabled or uninstalled PHP module

📌 Example of the Error

Fatal error: Uncaught Error: Call to undefined function mysql_connect() in C:\xampp\htdocs\test.php:3

🛠️ Step-by-Step Fix

✅ Step 1: Identify the Function Name

In the error above, the problem is with mysql_connect().

✅ Step 2: Understand Why It’s Failing

mysql_connect() was removed in PHP 7. Use mysqli_connect() or PDO instead.

✅ Step 3: Replace Deprecated Functions

Update your code with modern and supported functions.

🔁 Fixed Code Example

Here’s the corrected version using mysqli_connect():

<?php
$host = "localhost";
$username = "root";
$password = "";
$database = "test_db";

// Create connection
$conn = mysqli_connect($host, $username, $password, $database);

// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}
echo "Connected successfully!";
?>

💡 Common Fixes for Similar Errors

• Missing File Include: Add include 'file.php'; if the function is in another file.
• Typo in Function: Check for spelling mistakes like strlent() instead of strlen().
• Missing PHP Extension: Use phpinfo() to check if required extensions are loaded.

✅ Complete Copy-Paste PHP Code to Run Locally

Paste this in your C:/xampp/htdocs/test.php file and run via http://localhost/test.php:

<?php
$host = "localhost";
$username = "root";
$password = "";
$database = "test_db";

// Try connecting using mysqli
$conn = mysqli_connect($host, $username, $password, $database);

if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
} else {
    echo "Connected successfully!";
}
?>

❓ Frequently Asked Questions (FAQs)

1. Why am I getting "Call to undefined function" in PHP?

This happens when you use a function that doesn't exist in your PHP environment. It could be due to a typo, missing extension, or deprecated function.

2. How to fix "mysql_connect()" error in PHP 8?

mysql_connect() is removed in PHP 7+. Use mysqli_connect() or PDO instead.

3. How do I know which PHP functions are enabled?

Create a file with <?php phpinfo(); ?> and open it in browser. Look for loaded extensions.

4. What if the function exists but still gives the error?

Make sure the PHP module or library for that function is installed and enabled in php.ini.

📚 Conclusion

The PHP Fatal Error: Call to Undefined Function issue is common but easy to fix with the right steps. Always stay updated with the PHP version you're using and avoid deprecated functions.

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