a, b, c, ....

b = 1, 2

a, 1, 2, c, ...

a, 1,, 2, c, ...

list = replace(commaSplit(replace("a, 1,, 2, c, ...", ",," => "protectorUnique")), "protectorUnique" => ",")

1 = у, ф

a, у, ф,, 2, c, ...

0 => a
1 => 
    0=> 
        0 => у 
        1 => ф
    1 => 2
2 => c
3 => ...

0 => a
1 => 
    0 => у 
    1 => ф
    2 => 2
2 => c
3 => ...

0 => a
1 => 
    0=> 
        0 => у 
        1 => ф
    1 => 2
2 => c
3 => ...

1,1 => 1,(1,1) => 1,1,,1 => 1,1,,(1,1) => 1,1,,1,,,1 => 1,(1,1),,1,,,1 => 1,1,,1,,1,,,1 => err

// таблица замен
, => @zpt
( => @obr
) => @cbr

// наращивание сетки

1@zpt @obr1@cbr, 2@zpt @obr2@cbr 
   => 1@zpt @obr1@cbr, (1@zpt @obr1@cbr, 2@zpt @obr2@cbr)
   => 1@zpt @obr1@cbr, ((1@zpt @obr1@cbr, 2@zpt @obr2@cbr), 2@zpt @obr2@cbr)

// резульат парсинга

0 => 1, (1) 
   0 =>
      0 => 1, (1)
      1 => 2, (2)
   1 => 2, (2)

// таблица замен
, => \,
( => \(
) => \)

// таблица замен
, => \,
( => \(
) => \)
\ => \\

"x,<1,input/, textarea and button>,y,z"

$input = "x,<1,$input/, textarea and button>,y,z"

$input = "x,<1,x/,/<1/,///, textarea and button/>/,y/,z/, textarea and button>,y,z"

$input = "";
$input = "x,<1,".escape_str($input)."/, textarea and button>,y,z";
$input = "x,<1,".escape_str($input)."/, textarea and button>,y,z";
$input = "x,<1,".escape_str($input)."/, textarea and button>,y,z";
$input = "x,<1,".escape_str($input)."/, textarea and button>,y,z";
$input = "x,<1,".escape_str($input)."/, textarea and button>,y,z";

print_r($input);


function parse(&$arr, &$str, $start = 0){
  $el = 0;
  $arr[$el]="";
  while ($start<count($str)){
    switch($str[$start]){
      case "/" : $arr[$el] .= $str[++$start]; break;
      case "," : $arr[++$el]=""; break;
      case "<" : $arr[$el] = array(); $start = parse($arr[$el], $str, ++$start); break;
      case ">" : return $start; break;
      default  : $arr[$el] .= $str[$start];
    }
    $start++;
  }
  return $start;
}

$arr = array();
parse($arr, str_split($input));
print_r($arr);

x,<1,x/,/<1/,x///,///<1///,x///////,///////<1///////,x///////////////,///////////////
<1///////////////,///////////////////////////////, textarea and button///////////////>
///////////////,y///////////////,z///////////////, textarea and button///////>
///////,y///////,z///////, textarea and button///>///,y///,z///, textarea and button/>
/,y/,z/, textarea and button>,y,zArray
(
    [0] => x
    [1] => Array
        (
            [0] => 1
            [1] => x,<1,x/,/<1/,x///,///<1///,x///////,///////<1///////,///////////////, textarea and button///////>
                      ///////,y///////,z///////, textarea and button///>///,y///,z///, textarea and button/>
                      /,y/,z/, textarea and button>,y,z, textarea and button
        )

    [2] => y
    [3] => z
)

<![CDATA[<sender>John Smith</sender>]]>

<sender>John Smith</sender>

x,<1,x/,/<1/,x///,///<1///,x///////,///////<1///////,x///////////////,///////////////
<1///////////////,///////////////////////////////, textarea and button///////////////>
///////////////,y///////////////,z///////////////, textarea and button///////>
///////,y///////,z///////, textarea and button///>///,y///,z///, textarea and button/>
/,y/,z/, textarea and button>,y,z

(a, "1,2", c, ....)

(a, ((у, ф,), 2), c, ...) => (1, ((2, 3,), 4), 5, ...) // это адреса в таблице первого прохода