PHP - sumar por si mimos N veces

$a=$_GET['cantidad'];

$b=$_GET['veces'];

  echo "<table>";

  echo "<tr>";//<tr> ;

  for ($r=$a; $r<=$b; $r++);

   while ($cont =$b);

  echo $r;

  echo "</table>";//FINALIZA LA TABLA

<?php $a=2;

$b=6;

for ($r=0; $r<=$b; $r++){

    echo $a. '</br>';

    $a = $a + $a;

}

<?php $a=5;

$b=6;

$c = 0;

for ($r=0; $r<=$b; $r++){

    if ($c == 0){

        $c = $a;

    } else {

        echo $c . '</br>';

        $c = $c + $a;

    }

}

?>

$c = $a * $b;

echo $c;