Address review comments

Author: Girgias

main/SAPI.c

@@ -117,16 +117,15 @@ PHP_FUNCTION(header_register_callback)
 
 	if (ZEND_FCC_INITIALIZED(SG(send_header_fcc))) {
 		zend_fcc_dtor(&SG(send_header_fcc));
-		SG(send_header_fcc) = empty_fcall_info_cache;
 	}
 
 	/* Don't store callback if headers have already been sent:
 	 * It won't get used and we won't have a chance to release it. */
 	if (UNEXPECTED(SG(headers_sent))) {
 		zend_release_fcall_info_cache(&fcc);
+	} else {
+		zend_fcc_dup(&SG(send_header_fcc), &fcc);
 	}
-
-	zend_fcc_dup(&SG(send_header_fcc), &fcc);
 	RETURN_TRUE;
 }
 /* }}} */

tests/basic/header_register_callback_trampoline_after_headers_sent.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Test header_register_callback
+--FILE--
+<?php
+class TrampolineTest {
+    public function __call(string $name, array $arguments) {
+        echo 'Trampoline for ', $name, PHP_EOL;
+    }
+}
+$o = new TrampolineTest();
+$callback = [$o, 'trampoline'];
+
+echo "Send headers.\n";
+header_register_callback($callback);
+?>
+--EXPECT--
+Send headers.