Change Wordpress final output. LazyLoad integration for IMG tags. Not finished

functions.php

function callback($buffer) {
    $patterns = [
        '/<img(.*) src=/',
        '/<img(.*) class="/'
    ];
    $replacements = [
        '<img${1} data-src=',
        '<img${1} class="b-lazy '
    ];

    $buffer = preg_replace($patterns, $replacements, $buffer);

    return $buffer;
}

function buffer_start() {
    ob_start("callback");
}
add_action('wp_head', 'buffer_start');

function buffer_end() {
    ob_end_flush();
}
add_action('wp_footer', 'buffer_end');