Ajax搜索POST到php

Question

I need some help with refining my current search.

I have folder with images that are named as:

20171116-category_title.jpg       (where first number is date yyyymmdd)

My current search looks like this:

<?php
// string to search in a filename.

if(isset($_POST['question'])){
    $searchString = $_POST['question'];
}
// image files in my/dir
$imagesDir = '';
$files = glob($imagesDir . '*.{jpg,jpeg,png,gif}', GLOB_BRACE);

// array populated with files found 
// containing the search string.
$filesFound = array();

// iterate through the files and determine 
// if the filename contains the search string.
foreach($files as $file) {
    $name = pathinfo($file, PATHINFO_FILENAME);

    // determines if the search string is in the filename.
    if(strpos(strtolower($name), strtolower($searchString))) {
         $filesFound[] = $file;
    } 
}

// output the results.
echo json_encode($filesFound, JSON_UNESCAPED_UNICODE);
?>

And this works just fine but...

1. I would like to limit search only to part of .jpg name that contains "title" behind underscore " _ " and after that (if possible) to expand search to:

2. To make double search if AJAX POST sends following format: abc+xyz where delimiter "+" practicaly means 2 queries.

   First part is (abc) which targets "category" that stands between minus and underscore and second part of query (xyz) (which is basically my first question) only among previously found (category) answers.

   Your tips are more than welcome! Thank you!