$string = '<table border=\'0\'       cellpadding  = "0" cellspacing=0 style= "border-collapse: collapse">';
if (preg_match_all('#\s+([^=\s]+)\s*=\s*((?(?="|\') (?:"|\')([^"\']+)(?:"|\') | ([^\s]+)))#isx', $string, $matches)) {
  print_r($matches);
}
Array ( [0] => Array ( [0] => border='0' [1] => cellpadding = "0" [2] => cellspacing=0 [3] => style= "border-collapse: collapse" ) [1] => Array ( [0] => border [1] => cellpadding [2] => cellspacing [3] => style ) [2] => Array ( [0] => '0' [1] => "0" [2] => 0 [3] => "border-collapse: collapse" ) [3] => Array ( [0] => 0 [1] => 0 [2] => [3] => border-collapse: collapse ) [4] => Array ( [0] => [1] => [2] => 0 [3] => ) )