$circle_num = 3;
$radius = 100/$circle_num;
$svg = '<svg height="100" width="100">';
for ($i = 1; $i <= $circle_num; $i++){
$rad = $radius*$i
$svg .= '<circle cx="50" cy="50" r="'.$rad .'" stroke="black" stroke-width="2" fill="transparent"/>';
}
$svg = '</svg>';
echo $svg;