Get Field and Update ACF Field on Post Save

gistfile1.txt

// In this example we are getting a checkbox and dropdown field (noted below) and then calculating the sum. Then, we update a custom field with the total.

add_action( 'save_post', 'custom_field_calc', 20, 2 );

function custom_field_calc( $id, $post ) {
    // bail out if this is an autosave
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }
 
    // bail out if this is not a catalog item
    if ( $post->post_type != 'custom-cpt' ) { // Put your Custom Post Type slug here
        return;
    }
 
    // ... do override stuff

    global $post;
    $post_id=$post->ID;

    $group = get_field('mygroup'); // Here we are using a Group field, below are the group's subfields
    $myfield1 = $group['field1'];
    $myfield2 = $group['field2'];

    $myDropdown = get_field('my_dropdown'); // This is a dropdown field and we're using a switch to set the value based on the selection.
    switch ($myDropdown) {
        case "Option 1":
            $dropdownValue = '0';
            break;
        case "Option 2":
            $dropdownValue = '10';
            break;
        case "Option 3":
            $dropdownValue = '20';
            break;
        default:
            $dropdownValue = '0';
    }
    
    $totalValue = $myfield1 + $myfield2 + $myDropdown; // Calculate everything from above
    update_post_meta($post_id, 'fieldToUpdate', $totalValue); // update a custom field

}