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I'm using ACF (Advance Custom Fields) plugin in my theme. This plugin provides a simpler methods get_field() which we can use in place of WordPress native get_post_meta() method. Now, the issue is the get_field() method works fine if ACF plugin is active, but when the plugin is not active the theme front-end throughs the following error:

Fatal error: Call to undefined function get_field()

Now, I tried to provide a fallback function, which should be used when the plugin is not active, my fallback function is below:

if ( !function_exists('get_field') ) {

    function get_field($key) {
        return get_post_meta(get_the_ID(), $key, true);
    }
}

add_action( 'after_setup_theme', 'get_field' );

Now, this function do work when the ACF plugin is not active but as soon as I try to activate the ACF plugin the backend throughs an error that:

Fatal error: Cannot redeclare get_field()

Now, I want to use my fallback function ONLY when plugin is not active and it shoud not stop activation of the ACF plugin afterwards.

What could be the solution?

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The after_setup_theme hook is actually an action hook not a filter hook. Nonetheless that should not be needed.

Give this a try:

if( ! function_exists( 'get_field' ) ) {
    function get_field( $key, $post_id = false, $format_value = true ) {
        if( false === $post_id ) {
            global $post;
            $post_id = $post->ID;
        }

        return get_post_meta( $post_id, $key, true );
    }
}
  • No, the plugin activation still throws Cannot redeclare get_field() error. – Faisal Khurshid Oct 28 '16 at 11:10
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The issue has been solved. It seems ACF plugin defined get_field() function is not able to override the theme defined get_field() function on ACF plugin activation. Therefore, it throws Fatal error: Cannot redeclare get_field() error if we use the following code:

if ( !function_exists('get_field') ) {

    function get_field($key) {
        return get_post_meta(get_the_ID(), $key, true);
    }
}

Now since, before activating ACF plugin, we only need this custom wrapper function in front-end of our theme we can limit it's registration only to the front-end like this:

if ( !is_admin() && !function_exists('get_field') ) {

    function get_field($key) {
        return get_post_meta(get_the_ID(), $key, true);
    }

}

Now, our backend is not aware of our custom defined get_field() function so it won't cause any issue while activating the plugin and after activating the plugin the ACF plugin's get_field() function will take over as expected.

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this answers are not good. as get_field does a lot more than getting post_meta it also gets options. best bet is for you to shim the get_field function from core into the theme using the if ( !is_admin() && !function_exists('get_field') ) {} you may have to modify certain things the function is doing such as calling other acf hooks and functions. My preferred options is to not use get_field and use the wp core functions to get the post, comment,user meta and the get_option function. it will decouple your front end from the plugin as well as speed it up.

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