PHP foreach performance when adding string to variable
foreach($data as $r=>$d)
          {
            $return = $return. "<tr>
            <td>
            ".$d["client_id"]."
            </td>
        ......
            <td>
                ".$d["date_stamp"]."

            </td>
            </tr>";
            }
          }

foreach($data as $r=>$d)
        {
          $now= "<tr>
          <td>
          ".$d["client_id"]."
          </td>
      ......
          <td>
              ".$d["date_stamp"]."

          </td>
          </tr>";
          $return = $return.$now;
        }

//////////function to get time
    function parsemicrotime(){
       list($usec, $sec) = explode(" ",microtime());
       return ((float)$usec + (float)$sec);
       }

    ////////////define test array
    $a = array();
    for($i = 0; $i < 5000; $i ++)//generate 5k rows
      {
        for($k=0; $k<5;$k++)//lets have just 6 columns
          {
            $a[$i]["column_".$k] = 'test string '.$i.' / '.$k.' - note that the size of the $output string makes a huge difference ';
          }
      }

    ///////////////first test
    $time_start = parsemicrotime();
        $output = '';
        foreach($a as $row=>$columns)
          {
            $output = $output ."
            <tr>
              <td>".$columns["test_0"]. "</td>
              <td>" .$columns["test_1"]. "</td>
              <td>" .$columns["test_2"]. "</td>
              <td>" .$columns["test_3"]. "</td>
              <td>" .$columns["test_4"]. "</td>
              <td>" .$columns["test_5"]. "</td>
            </tr>";
          }
    $approach_1_result = parsemicrotime()-$time_start;


    /////////////second test
    $time_start2 = parsemicrotime();
        $output2 = '';
        foreach($a as $row2=>$columns2)
          {
            $now2= "
            <tr>
              <td>".$columns["test_0"]. "</td>
              <td>" .$columns["test_1"]. "</td>
              <td>" .$columns["test_2"]. "</td>
              <td>" .$columns["test_3"]. "</td>
              <td>" .$columns["test_4"]. "</td>
              <td>" .$columns["test_5"]. "</td>
            </tr>";
            $output2 = $output2 .$now2;
          }
    $approach_2_result = parsemicrotime()-$time_start2;


    /////////////third test
    $time_start3 = parsemicrotime();
    ob_start();
        $output3 = '';
        foreach($a as $row3=>$columns3)
          {
            echo "
            <tr>
              <td>".$columns["test_0"]. "</td>
              <td>" .$columns["test_1"]. "</td>
              <td>" .$columns["test_2"]. "</td>
              <td>" .$columns["test_3"]. "</td>
              <td>" .$columns["test_4"]. "</td>
              <td>" .$columns["test_5"]. "</td>
            </tr>";
          }
    $output3 = ob_get_clean();
    $approach_3_result = parsemicrotime()-$time_start3;


    die("first test:".$approach_1_result."<br>second test:".$approach_2_result."<br>third test:".$approach_3_result);

$a = [];
for($i = 0; $i < 10000; $i ++)
  $a[] = $i;

$output = '';
foreach($a as $k => $v)
  $output = $output . "some static text" . $v . "some other text";

$output .= 'some static text' . $v . 'some other text';

$output = '';
ob_start();
foreach($a as $k => $v)
  echo 'some static text' . $v . 'some other text';
$output = ob_get_clean();

ob_start();
foreach($data as $r=>$d)
{
    echo "<tr>
    <td>
    ";
    echo $d["client_id"];

    echo "</td>
    ......
    <td>
        ";
    echo $d["date_stamp"];

    echo "</td>
    </tr>";
}

$return .= ob_get_contents();
ob_end_clean();