Add Header::RFC7231 constant to avoid hardcoded format string

Author: koriym

src/DevEtagSetter.php

@@ -29,7 +29,7 @@ public function __invoke(ResourceObject $ro, int|null $time = null, HttpCache|nu
         // This is useful for debugging purposes.
         // Usually, the ETag is a hash of the resource view or body.
         $ro->headers[Header::ETAG] = $uriEtag;
-        $ro->headers[Header::LAST_MODIFIED] = gmdate('D, d M Y H:i:s \G\M\T', 0);
+        $ro->headers[Header::LAST_MODIFIED] = gmdate(Header::RFC7231, 0);
         $this->setCacheDependency($ro);
     }
 

src/EtagSetter.php

@@ -37,7 +37,7 @@ public function __invoke(ResourceObject $ro, int|null $time = null, HttpCache|nu
 
         $etag =  $this->getEtag($ro, $httpCache);
         $ro->headers[Header::ETAG] = $etag;
-        $ro->headers[Header::LAST_MODIFIED] = gmdate('D, d M Y H:i:s \G\M\T', $time);
+        $ro->headers[Header::LAST_MODIFIED] = gmdate(Header::RFC7231, $time);
         $this->setCacheDependency($ro);
     }
 

src/Header.php

@@ -17,5 +17,12 @@ final class Header
     public const CACHE_CONTROL = 'Cache-Control';
     public const AGE = 'Age';
     public const LAST_MODIFIED = 'Last-Modified';
+
+    /**
+     * RFC 7231 date format for HTTP headers
+     *
+     * @see https://www.rfc-editor.org/rfc/rfc7231#section-7.1.1.1
+     */
+    public const RFC7231 = 'D, d M Y H:i:s \G\M\T';
     public const HTTP_IF_NONE_MATCH = 'HTTP_IF_NONE_MATCH';
 }