Find number is prime or not using PHP

<?php
$num = 17;
$prime=true;
for($i=2;$i<=$num/2;$i++)
{
    $j = $num % $i;
    if($j==0){
        $prime=false;
        break;
    }
}
if($prime==true){
    echo "No is prime<br/>";
}
else{
    echo "No is not prime";
}

?>