If you’re developing custom WordPress plugins and encounter this error in your logs:

PHP Fatal error: Uncaught Error: [] operator not supported for strings in /path/to/plugin.php:104

You’re likely trying to append to a variable that’s a string instead of an array.

## The Problem

This error commonly occurs when using WordPress filters that expect arrays. Here’s the problematic code:

function my_custom_function($post_types) {

$post_types[] = 'my_custom_post_type'; // Fatal error if $post_types is a string!

return $post_types;

}

add_filter('some_filter', 'my_custom_function');

The issue is that some WordPress filters might pass a **string** or **null** instead of an array, causing PHP to throw a fatal error when you try to use the `[]` array append operator.

## The Solution

Always validate that the parameter is an array before trying to append to it:

function my_custom_function($post_types) {

// Ensure $post_types is an array

if (!is_array($post_types)) {

$post_types = array();

}

$post_types[] = 'my_custom_post_type';

return $post_types;

}

add_filter('some_filter', 'my_custom_function');

Real-World Example: Jetpack Stats Integration

I encountered this error while adding Jetpack Stats support to a custom post type. Here’s the fixed code:

// Enable Jetpack Stats for custom post type

function wpm_jetpack_stats_post_types($post_types) {

// Ensure $post_types is an array

if (!is_array($post_types)) {

$post_types = array();

}

if (!in_array('watch_project', $post_types)) {

$post_types[] = 'watch_project';

}

return $post_types;

}

add_filter('stats_array', 'wpm_jetpack_stats_post_types');

add_filter('jetpack_relatedposts_filter_post_type', 'wpm_jetpack_stats_post_types');

Best Practices

When working with WordPress filters and hooks:

1. Always validate input types – Don’t assume filters will pass the expected data type
2. Use `is_array()`checks before array operations
3. Check for duplicates with `in_array()` to avoid adding the same value multiple times
4. Test thoroughly – Check your error logs regularly during development

Key Takeaways

• The `[]` operator only works on arrays, not strings or other types
• WordPress filters can sometimes pass unexpected data types
• Always add type checking before manipulating filter parameters
• A simple `is_array()` check can prevent fatal errors

This small defensive coding practice will save you from mysterious fatal errors and make your plugins more robust!