A formula to find prime numbers in a loop

Question

I need to find prime numbers with for loop or while loop

I wrote this but this is wrong

<?php
$i = 1;
while($i<5)
{
    for($j=1; $j<=$i; $j++)
    {
        if ($j != 1 && $j != $i)
        {
            echo $i . "/" . $j . "=" . $i%$j . "<br />";
            if ($i%$j != 0)
            {
                echo $i . "<br />";
            }
        }
    }
    echo "<br />";
    $i += 1;
}
?>

Is there a way to divide a number with an array to find the remaining?

Answer 1

$num = 25;
echo "Value Hardcored ".$num."<br>";

for($i=2; $i<$num; $i++)
 {
  if($num%$i==0)
  {
   $Loop = true;
   echo "This is Composite Number";
   break; 
  }
  $Loop = false; 
 }

 if($Loop == false)
 {
  echo "Prime Number";
 }

Answer 2

Here is a one-liner I found a while back to check for primes. It uses tally marks (unary math) to determine:

function is_prime_via_preg_expanded($number) {
    return !preg_match('/^1?$|^(11+?)\1+$/x', str_repeat('1', $number));
}

Check all numbers sequentially for primes:

$i=2; // start here (2 is the first prime)
while (1) { // neverending loop
    if (is_prime_via_preg_expanded($i)) echo $i." <br />\n";
    $i++;
}

To check only a range of numbers for primes like in the provided example:

$start = 2; // start here (2 is the first prime)
$end = 100;

$i=$start;
while ($i<=$end) {
    if (is_prime_via_preg_expanded($i)) echo $i." <br />\n";
    $i++;
}