find text and replace in file using php

<?php
    $filename = "path/file.php";
    $code_add = file_get_contents(__DIR__."/views/code.php");
    $string_to_replace = "<?php if(get_option('allow_registration') == 1) { ?>";
    $replace_with = "<?php if(get_option('allow_registration') == 1) { ?>\n".$code_add;
    $content = file_get_contents($filename);
    $content_chunks = explode($string_to_replace, $content);
    $content = implode($replace_with, $content_chunks);
    file_put_contents($filename, $content);

    $css_add = "end of the line add";
    file_put_contents($filename, $css_add . PHP_EOL, FILE_APPEND);
?>