$re = '/"?(\w+)=\'?([\w]+)\'?"?|"?(\w+)="([\w\s]+)""?/';
$str = 'foo a=\'b\' c="d and e" wombat=true a fizz=\'buzz\' "hello=\'goodbye\'"';
$subst = "\"$1$3='$2$4'\"";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php