Determine if a Number is Prime

eratosthenes_sieve.php

<?php
function eratosthenes_sieve(&$sieve, $n) {
 $i = 2;

 while ($i <= $n) {
  if ($sieve[$i] == 0) {
   echo $i;

   $j = $i;
   while ($j <= $n) {
    $sieve[$j] = 1;
    $j += $i;
   }
  }

 $i++;
 }
}

$n = 100;
$sieve = array_fill(0, $n, 0);

// 2, 3, 5, 7, ..., 97
eratosthenes_sieve($sieve, $n);
?>

prime_v1.php

<?php
 function isPrime($n) {
  $i = 2;
  
  if ($n == 2) {
   return true;	
  }
 
  while ($i < $n) {
   if ($n % $i == 0) {
    return false;
   }
   $i++;
  }
 
  return true;
 }
 
 // prints the prime numbers between 2 and 100
 // 3, 5, 7, ..., 97
 for ($i = 3; $i < 100; $i++) {
  if (isPrime($i)) {
   echo $i;
  }
 }
?>

prime_v2.php

<?php
 function isPrime($n) {
  $i = 2;
 
  if ($n == 2) {
   return true;	
  }
 
  while ($i <= $n/2) {
   if ($n % $i == 0) {
    return false;
   }
   $i++;
  }
 
  return true;
 }
?>

prime_v3.php

<?php
 function isPrime($n) {
  $i = 2;
 
  if ($n == 2) {
   return true;	
  }
 
  $sqrtN = sqrt($n);
  while ($i <= $sqrtN) {
   if ($n % $i == 0) {
    return false;
   }
   $i++;
  }
 
  return true;
 }
?>