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Troubleshooting Common 'Invalid argument supplied for foreach()' Errors in Meta Box

40 threads Sep 10, 2025 Plugin → Meta box

Content

One of the most frequent errors encountered by users of the Meta Box plugin is the PHP warning: Invalid argument supplied for foreach(). This error can appear in various files and lines, often disrupting the WordPress admin area or front-end with multiple warnings. This guide will explain why this happens and provide the most effective solutions to resolve it.

Why This Error Occurs

The foreach() loop in PHP is designed to iterate over arrays. This error is triggered when the variable passed to the loop is not an array or is null. In the context of Meta Box, this almost always relates to the rwmb_meta_boxes filter. This filter is used to register custom meta boxes, and it must return an array of meta box definitions. If the filter returns anything other than an array (like a null value or a string), or if the code that applies the filter does not properly handle an empty result, the subsequent foreach loop will fail with this warning.

Common Solutions

1. Ensure Your Filter Returns an Array

The most common cause is a custom function hooked to the rwmb_meta_boxes filter that fails to return an array under certain conditions. Always ensure your function returns an array, even if it's empty.

add_filter( 'rwmb_meta_boxes', 'my_meta_boxes' );
function my_meta_boxes( $meta_boxes ) {
    // Your code to define meta boxes here
    
    // CRITICAL: Always return the array.
    return $meta_boxes;
}

2. Check for a Valid Array Before Looping

If you are modifying core plugin files (not recommended), or if the error points to a line in your theme, you can add a conditional check before the foreach loop. The best practice is to use is_array() or !empty().

// Example of a safe loop
$meta_boxes = apply_filters( 'rwmb_meta_boxes', array() );
if ( is_array( $meta_boxes ) && !empty( $meta_boxes ) ) {
    foreach ( $meta_boxes as $meta_box ) {
        new RW_Meta_Box( $meta_box );
    }
}

3. Check for Plugin or Theme Conflicts

As seen in the sample threads, this error can sometimes be caused by conflicts with other code, especially in multisite installations or when Meta Box is bundled with a theme. A standard troubleshooting step is to:

  1. Deactivate all other plugins except Meta Box.
  2. Switch to a default WordPress theme like Twenty Twenty-One.
  3. Check if the error persists. If it disappears, reactivate your plugins and theme one by one to identify the conflict.

4. Update Meta Box and Its Extensions

Many iterations of this error have been fixed in subsequent releases of the Meta Box plugin. Ensure you are running the latest version of both the core Meta Box plugin and any associated extensions (e.g., MB Elementor Integrator). The Meta Box team actively addresses these issues, and an update often provides the fix.

Conclusion

The Invalid argument supplied for foreach() warning is a common PHP error that points to a problem with the data structure being processed. By ensuring your custom code returns a proper array, adding safety checks, and keeping all components updated, you can efficiently eliminate this error from your site.

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