<?php

$re = '~(?:<\w+|(?!^)\G)(?:\s+(?:src|href)=(?:"[^"]*"|\'[^\']*\'))*\s+(?!(?:href|src)=)\w+=\K(?|"([^\s"=]*)"|\'([^\s\'=]*)\')~u';
$str = "<mytag src=\"src_here\" b=\"yes\" href=\"href_here\"> becomes <mytag src=\"src_here\" b=yes href=\"href_here\">\n<mytag b='yes'> becomes <mytag b=yes>\nbut <script>var b=\"yes\"</script> stays intact\n<mytag b=\"no no\" c=\"no=no\"> stays intact\n<tag href=\"something\"> text <tag src=\"dddd\"> intact"; 
$subst = "$1"; 
$result = preg_replace($re, $subst, $str);
echo $result;