Use /\?[^\"]+/
like bellow:
<?php
$string = '<a href="https://stackoverflow.com/title/tt3110958/?ref_=nm_flmg_act_2" style="color:#666" >';
$result = preg_replace("/\?[^\"]+/", "", $string);
echo $result;
?>
Output : <a href="https://stackoverflow.com/title/tt3110958/" style="color:#666" >
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solved How to remove query string from “href” attribute of anchor using php?