Remove string interpolation; it will be deprecated in PHP 8.2

Author: tvdijen

lib/CryptoEncoding/PEM.php

@@ -152,8 +152,11 @@ public function data(): string
      */
     public function string(): string
     {
-        return "-----BEGIN {$this->type}-----\n" .
-            trim(chunk_split(base64_encode($this->data), 64, "\n")) . "\n" .
-            "-----END {$this->type}-----";
+        return sprintf(
+            "-----BEGIN %s-----\n%s\n-----END %s-----",
+            $this->type,
+            trim(chunk_split(base64_encode($this->data), 64, "\n")),
+            $this->type
+        );
     }
 }