WordPress: PHP/JS For detecting CLS

cls.php

<?php

add_action( 'wp_head', function () {
    echo '<script>';
    echo "(function() {
    var cls = 0;

    new PerformanceObserver((entryList) => {
        var entries = entryList.getEntries();
        for (var i = 0; i < entries.length; i++) {
            var entry = entries[i];
            if (!entry.hadRecentInput) {
                cls += entry.value;
                console.log('Current CLS value:', cls, entry);
            }
        }
    }).observe({type: 'layout-shift', buffered: true});
})();
";
    echo '</script>';
}, 0 );