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How to concatenate CSV rows into new rows?
"1", "2", "3", "4"
 "5", "6", "7", "8"
 "9","10","11","12"
"13","14","15","16"
"17","18","19","20"
"21","22","23","24"
"25","26","27","28"
"29","30","31","32"
"33","34","35","36"

1,2,3,4,5,6,7,8,9,10,11,12
13,14,15,16,17,18,19,20,21,22,23,24
25,26,27,28,29,30,31,32,33,34,35,36

$path = "data.csv";
$row = 0;
$newrow = 0;
$newrows = array();
if (($handle = fopen($path, "r")) !== FALSE) {
  while (($data = fgetcsv($handle, 1000, ",")) !== FALSE) {
    $newrows[$newrow] = implode("," $data);
    if ($row % 3) $newrow++;
    $row++;
  }
  fclose($handle);
}

$newrows = array (
  [0] = "1,2,3,4,5,6,7,8,9,10,11,12",
  [1] = "13,14,15,16,17,18,19,20,21,22,23,24",
  [2] = "25,26,27,28,29,30,31,32,33,34,35,36"
)

array (
  [0] => 1,2,3,4,13,14,15,16,25,26,27,28
  [1] => 5,6,7,8,17,18,19,20,29,30,31,32
  [2] => 9,10,11,12,21,22,23,24,33,34,35,36
)

$path = "data.csv";
$row = 1;
if (($handle = fopen($path, "r")) !== FALSE) {
  while (($data = fgetcsv($handle, 1000, ",")) !== FALSE) {
    for ($i = 1; $i <= 3; $i++) {
      if ($row % $i == 0) $newrows[$i] .= implode(",", $data);
    }
    $row++;
  }
}

print_r($newrows);

Array (
  [1] => 1,2,3,45,6,7,89,10,11,1213,14,15,1617,18,19,2021,22,23,2425,26,27,2829,30,31,3233,34,35,36
  [2] => 5,6,7,813,14,15,1621,22,23,2429,30,31,32
  [3] => 9,10,11,1221,22,23,2433,34,35,36
)

$newrow = 0; $row = 1;
while (($data = fgetcsv($handle, 1000, ",")) !== FALSE) {
    if( !isset( $newrows[$newrow])) $newrows[$newrow] = '';
    $newrows[$newrow] .= implode(",", $data);
    if ($row % 3 == 0) {
        $newrow++;
    } else {
        $newrows[$newrow] .= ', ';
    }
    $row++;
}