diff --git a/euler21.php b/euler21.php
new file mode 100644
index 0000000..3d87337
--- /dev/null
+++ b/euler21.php
@@ -0,0 +1,21 @@
+<?php
+/*
+* Gnieark's anwser to euler problem 21
+* https://projecteuler.net/problem=21
+*/
+include("_functions.php");
+
+$sumOfDivisors=array();
+for($i=1;$i<10000;$i++){
+  $sumOfDivisors[$i]=array_sum(getAllDivisors($i,true,false));
+}
+$sum=0;
+//If d(a) = b and d(b) = a, where a ≠ b,
+
+foreach($sumOfDivisors as $a => $d){
+
+  if((isset($sumOfDivisors[$d])) && ($sumOfDivisors[$d]==$a) && ($a<>$d)){
+    $sum+=$a;
+  }
+}
+echo $sum;
\ No newline at end of file