update after feedback

exercises/practice/atbash-cipher/.meta/example.php

@@ -29,19 +29,15 @@ function encode($string)
 
 function decode($string)
 {
-    $a_z = range('a', 'z');
-    $z_a = range('z', 'a');
-
+    $decodeMap = array_combine(range('z', 'a'), range('a', 'z'));
     $encodedString = str_replace(' ', '', $string);
 
     $decoded = [];
     foreach (str_split($encodedString) as $char) {
-        // Check if the character is numeric
-        if ($char >= '0' && $char <= '9') {
+        if (isset($decodeMap[$char])) {
+            $decoded[] = $decodeMap[$char];
+        } elseif ($char >= '0' && $char <= '9') {
             $decoded[] = $char;
-        } elseif ($char >= 'a' && $char <= 'z') {
-            // Map it from z_a back to a_z
-            $decoded[] = $a_z[array_search($char, $z_a)];
         }
     }
 

exercises/practice/atbash-cipher/AtbashCipher.php

@@ -28,3 +28,8 @@ function encode(string $text): string
 {
     throw new \BadFunctionCallException("Implement the encode function");
 }
+
+function decode(string $text): string
+{
+    throw new \BadFunctionCallException("Implement the decode function");
+}