echo "<p>You're currently filtering by "" . implode(", ", array_keys($product_filter)) . ""</p>";
That code part gives the opportunity for malicious user to injects JavaScript code into the filter parameter (e.g., via a URL query string like ?filter=<script>alert('XSS')</script>), it would be executed in the browser because the input is not sanitized or escaped.
filterparameter (e.g., via a URL query string like?filter=<script>alert('XSS')</script>), it would be executed in the browser because the input is not sanitized or escaped.