using a loop inside a variable PHP

You can just save whatever string you generate in the loop in a variable and then concatenate it.

$str_buffer = '';
$foo =  while(somethign here too){ 
   $str_buffer.= "<tr class='table_headsnonw' style='font-size:12px;'>
                      <td>1</td>
                      <td>
                          3
                      </td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                  </tr>";
 }



$variable="stuff".$str_buffer." other static stuff";
echo $variable;

You should concat, e.g:

$variable="
<table style='margin-bottom:30px;'>
                  <tr class='table_heads'>
                      <b><td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>S.No</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Ingredients</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Weight <small><i> - (In Grams)</i></small></td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Fat</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Sweetness</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Solid 1</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Solid 2</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Anti-Freezing Power</td>
                 </tr>";
 while(somethign here too){ // here is the problem even i try to concenite 
   $variable .= "<tr class='table_headsnonw' style='font-size:12px;'>
                      <td>1</td>
                      <td>
                          3
                      </td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                  </tr>";
 }

This is maybe not a good approach to write code. You should echo php inside html and not the opposite. Anyway, to fix your issue:

$variable="
<table style='margin-bottom:30px;'>
                  <tr class='table_heads'>
                      <b><td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>S.No</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Ingredients</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Weight <small><i> - (In Grams)</i></small></td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Fat</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Sweetness</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Solid 1</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Solid 2</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Anti-Freezing Power</td>
                  </tr>";


 while(somethign here too){ 
$variable .="       <tr class='table_headsnonw' style='font-size:12px;'>
                      <td>1</td>
                      <td>
                          3
                      </td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                      <td>300</td>
                  </tr>";
 }


$variable .="<tr class='table_heads'>
    <b><td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>S.No</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Ingredients</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>Weight <small><i> - (In Grams)</i></small></td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>0</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>0</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>0</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>0</td>
                      <td style='border: 1px solid #d8d8d8;padding: 2px 7px 2px 7px;background: #ddd;font-weight: bold;font-size: 11px;'>0</td>
                  </tr>

 ";

Do you see that I used .= to append to the variable? This will add to your variable something from each iteration of the loop and then will add the last part after.

In a while loop, the basic code should be something like

$x=1
$var="<table>"; 
// here you can add a more "outside loop" rows
// like $var.="<tr><td>whatever content</td></tr>";

// then create content from the loop    
while($x <= 5) {
    $var.="<tr><td>$x</td></tr>";
    $x++;
}
// and close the table outside the loop
$var.="</table>";
echo $var;

Supposing you have to loop an array, the basic structure of your code should be something like:

$array=array("a", "b", "c");

$var="<table>"; 
// here you can add a more "outside loop" rows
// like $var.="<tr><td>whatever content</td></tr>";

// then create content from the loop
foreach ($array as $item) {
    $var.="<tr><td>$item</td></tr>";
}   
// close the table outside the loop
$var.="</table>";
echo $var;