index.php

<?php

require "searchfile.php";

$word   = 'palavra';
$files = array('servicos.php', 'contato.html');

$search = new SearchFile($word, $files); 
print_r($search->getResults());

searchfile.php

<?php 

class SearchFile
{
    private $files;
    private $search;
    private $results;

    public function __construct($search, $files=array())
    {
        $this->search = $search;
        $this->files = $files;
        $this->execute();
    }

    public function execute()
    {
        foreach ($this->files as $file) {
            if (file_exists($file)) {
                $string = file_get_contents($file);
                if (stristr($string, $this->search) == true) {
                    $this->results[] = $file;
                }
            }
        }
    }

    public function getResults()
    {
        return $this->results;
    }
}