fix: Usage of header function

The header function of PHP have only an argument to define the header
name and value. This argument need to be a string.

More info: https://www.php.net/header

Signed-off-by: Vitor Mattos <vitor@php.rio>

tests/server/multirequest.php

@@ -1,5 +1,5 @@
 <?php
 
-header("content-type", "text-plain");
+header('content-type: text/plain');
 sleep(1 + rand(0, 3));
 echo $_REQUEST["param"];