If you want to find any given file inside a folder and its subfolders, you can do that with the code below.

<?php
//=========== find a file based on wildcard in a dir recursively ===========
clearstatcache();

$sourcepath = "/path_to_folder"; //eg. dirname(__FILE__);

// Replace \ by / and remove the final / if any
$root = ereg_replace( "/$", "", ereg_replace( "[\\]", "/", $sourcepath ));

// Search for text files
$results = m_find_in_dir( $root, "index.html" );
if( false === $results ) {
    echo "'{$sourcepath}' is not a valid directory\n";
} else {
    echo "<pre>";
    print_r( $results );
    echo "</pre>";
}

/**
 * Search for a file maching a regular expression
 *
 * @param string $root Root path
 * @param string $pattern POSIX regular expression pattern
 * @param boolean $recursive Set to true to walk the subdirectories recursively
 * @param boolean $case_sensitive Set to true for case sensitive search
 * @return array An array of string representing the path of the matching files, or false in case of error
 */
function m_find_in_dir( $root, $pattern, $recursive = true, $case_sensitive = false ) {
    $result = array();
    if( $case_sensitive ) {
        if( false === m_find_in_dir__( $root, $pattern, $recursive, $result )) {
            return false;
        }
    } else {
        if( false === m_find_in_dir_i__( $root, $pattern, $recursive, $result )) {
            return false;
        }
    }
   
    return $result;
}

/**
 * @access private
 */
function m_find_in_dir__( $root, $pattern, $recursive, &$result ) {
    $dh = @opendir( $root );
    if( false === $dh ) {
        return false;
    }
    while( $file = readdir( $dh )) {
        if( "." == $file || ".." == $file ){
            continue;
        }
        if( false !== @ereg( $pattern, "{$root}/{$file}" )) {
            $result[] = "{$root}/{$file}";
        }
        if( false !== $recursive && is_dir( "{$root}/{$file}" )) {
            m_find_in_dir__( "{$root}/{$file}", $pattern, $recursive, $result );
        }
    }
    closedir( $dh );
    return true;
}

/**
 * @access private
 */
function m_find_in_dir_i__( $root, $pattern, $recursive, &$result ) {
    $dh = @opendir( $root );
    if( false === $dh ) {
        return false;
    }
    while( $file = readdir( $dh )) {
        if( "." == $file || ".." == $file ){
            continue;
        }
        if( false !== @eregi( $pattern, "{$root}/{$file}" )) {
            $result[] = "{$root}/{$file}";
        }
        if( false !== $recursive && is_dir( "{$root}/{$file}" )) {
            m_find_in_dir__( "{$root}/{$file}", $pattern, $recursive, $result );
        }
    }
    closedir( $dh );
    return true;
}

?>

This will return any files, also ones based on wildcards, such as *.txt, inde?.html, etc.