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<?php
// Do NOT include the opening php tag

add_filter( 'stylesheet_uri', 'child_stylesheet_uri' );
/**
 * Cache bust the style.css reference.
 *
 */
function child_stylesheet_uri( $stylesheet_uri ) {
    return add_query_arg( 'v', filemtime( get_stylesheet_directory() . '/style.css' ), $stylesheet_uri );
}