convert_src_to_base64.php > newfile.html

<?php

function data_uri($filename) {
    $mime = mime_content_type($filename);
    $data = base64_encode(file_get_contents($filename));

    return "data:$mime;base64,$data";
}

$content = file_get_contents('oldfile.html');
$search = '/(<img\s+src=["\'])([^"\']+)(["\']\s+[^>]+>)/';
$content = preg_replace_callback($search, create_function(
        '$matches',
        'return $matches[1] . data_uri($matches[2]) . $matches[3];'
    ), $content);
echo $content;