Get value of attribute from a string

You will have to use regex. I'm not a regex expert but this works for me. You can test your regex easly on https://regex101.com/

$s = 'Lorem Ipsum is simply dummy text of the printing and typesetting industry.[fusion_faq filters="no" featured_image="no" cats_slug="unibet-india" exclude_cats="" orderby="date" order="DESC" type="" boxed_mode="" border_size="" border_color="" background_color="" hover_color="" divider_line="" title_font_size="16px" icon_size="22" icon_color="#0ead6a" icon_boxed_mode="no" icon_box_color="" icon_alignment="right" toggle_hover_accent_color="#0ead6a" hide_on_mobile="small-visibility,medium-visibility,large-visibility" class="" id="" /][/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]';

    $matches = array();
    preg_match('/cats_slug="(.*?)"/', $s, $matches);
    print_r($matches[1]);

This looks like shortcodes (or BBCodes). You should use a shortcodes parser:

Parser libraries:

• https://github.com/thunderer/Shortcode
• https://github.com/s9e/TextFormatter
• https://github.com/milesj/decoda

PHP extension:

• https://www.php.net/manual/en/book.bbcode.php