Why You Should Use function_exists() in WordPress Themes

When building or customizing a WordPress theme, it’s easy to assume every function you call will always be available. But here’s the catch — sometimes, a single missing function can break your entire website.

That’s why checking if a function exists before using it is one of the simplest yet most important best practices in WordPress development. It keeps your theme safe from fatal errors and ensures your site keeps running smoothly, even if something goes missing.

Let’s explore why this matters, how it works, and the right way to use it.

What Happens If You Don’t Check for a Function

Imagine your theme or plugin tries to call a function that doesn’t exist — maybe because:

• A plugin providing that function was deactivated or deleted.
• You renamed or removed the function during an update.
• Another plugin or theme conflict caused the function to be undefined.

When that happens, PHP throws a “Call to undefined function” fatal error, and your site can crash or show the dreaded white screen of death.

In short, a single missing function can take your site down.

The Safe Way: Use function_exists()

To prevent this, PHP gives us a built-in helper called function_exists().
This function checks whether a particular function is defined before executing it.

Here’s the syntax:

if ( function_exists( 'function_name' ) ) {
    // Safe to call the function
    function_name();
}

If the function is available, PHP runs it. If not, the code simply skips it — no errors, no crashes.

A Simple Example

Let’s say you created a custom function to show the current time with timezone:

function wp_passion_show_time_with_timezone() {
    $time = current_time( 'F j, Y g:i a e' );
    echo "<p>The current time is $time</p>";
}

You might want to call this function inside your theme’s template like this:

<?php wp_passion_show_time_with_timezone(); ?>

But what if someone removes or renames that function later? The site will break immediately.

The better and safer way is:

<?php
if ( function_exists( 'wp_passion_show_time_with_timezone' ) ) {
    wp_passion_show_time_with_timezone();
}
?>

Now, even if the function disappears, your site won’t go down — it’ll just skip that line.

Real-World Use Cases in WordPress Themes

Checking if a function exists isn’t just a habit — it’s a shield for your theme. Here’s where it helps the most:

1. When Your Theme Relies on a Plugin

If your theme uses plugin functions (like from WooCommerce, ACF, or Jetpack), those functions vanish if the plugin is inactive. Checking for them keeps your theme from breaking.

2. When Creating Child Themes

Child themes often override parent theme functions. Using function_exists() ensures your custom code only runs if the parent’s function doesn’t already exist — preventing “cannot redeclare” errors.

Example:

if ( ! function_exists( 'parent_theme_header' ) ) {
    function parent_theme_header() {
        // Your custom header code
    }
}

3. During Theme or Plugin Updates

Functions may be renamed, deprecated, or removed in updates. Wrapping them with existence checks future-proofs your code.

Best Practices for Using function_exists()

To make the most of this approach, keep these guidelines in mind:

• Always check before calling plugin or custom functions you didn’t define yourself.
• Prefix your functions with your theme or plugin name to prevent naming conflicts.
• Document dependencies in comments so others (and future you) know which plugins are required.
• Wrap conditionally — instead of checking every function individually, you can sometimes check once and wrap an entire block of related code.

Example:

if ( function_exists( 'woocommerce' ) ) {
    // Add your WooCommerce-related code here
}

Why This Tiny Step Makes a Big Difference

WordPress thrives on flexibility — themes, plugins, hooks, and filters all working together.
But that flexibility also means dependencies can change at any time.

Using function_exists():

• Prevents fatal errors when a function disappears
• Keeps your theme compatible with different environments
• Helps avoid plugin conflicts
• Makes your code cleaner, safer, and more professional

Checking if a function exists before using it might feel like a small detail, but it can save you from major headaches later.

Whether you’re developing a new theme, extending an existing one, or experimenting with snippets — make it a habit. It’s a simple, one-line safeguard that can keep your WordPress site running like a charm.