This error message is typically seen when using the add_filter() function in WordPress and is caused by passing an incorrect parameter as the callback function.

The add_filter() function is used to add a new filter to a specific filter hook, and the second parameter of the function is the callback function that will be called when the filter is applied. The callback function must be a callable, meaning it can be a function name, an anonymous function, or a class method.

In this case, the error message is saying that the second parameter passed to the add_filter() function is not a callable, but an array of mixed values. This means that the code is passing an array to the add_filter() function instead of a callable.

To fix this error, you need to pass a valid callable as the second parameter of the add_filter() function. If the error message is related to a plugin, you should check the documentation of the plugin to see what the expected parameter is.

Here is an example of a correct usage of the add_filter() function:

Copy codeadd_filter( 'the_title', 'my_custom_title_filter' );

function my_custom_title_filter( $title ) {
    return strtoupper( $title );

In this example, the filter ‘the_title’ is added and the callback function is ‘my_custom_title_filter’ which is a callable.

It’s important to make sure that the callback passed to the add_filter() function is correct, otherwise, it can cause unexpected behavior or bugs in your plugin or theme.

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