Data commit

Task/Emirp-primes/PHP/emirp-primes.php

@@ -0,0 +1,46 @@
+<?php
+
+function is_prime($n) {
+    if ($n <= 3) {
+        return $n > 1;
+    } elseif (($n % 2 == 0) or ($n % 3 == 0)) {
+        return false;
+    }
+    $i = 5;
+    while ($i * $i <= $n) {
+        if ($n % $i == 0) {
+            return false;
+        }
+        $i += 2;
+        if ($n % $i == 0) {
+            return false;
+        }
+        $i += 4;
+    }
+    return true;
+}
+
+function is_emirp($n) {
+    $r = (int) strrev((string) $n);
+    return (($r != $n) and is_prime($r) and is_prime($n));
+}
+
+$c = $x = 0;
+$first20 = $between = '';
+do {
+    $x++;
+    if (is_emirp($x)) {
+        $c++;
+        if ($c <= 20) {
+            $first20 .= $x . ' ';
+        }
+        if (7700 <= $x and $x <= 8000) {
+            $between .= $x . ' ';
+        }
+    }
+} while ($c < 10000);
+
+echo
+    'First twenty emirps :', PHP_EOL, $first20, PHP_EOL,
+    'Emirps between 7,700 and 8,000 :', PHP_EOL, $between, PHP_EOL,
+    'The 10,000th emirp :', PHP_EOL, $x, PHP_EOL;