Do not require hostname for non-persistent MySQL connection and require for persistent

src/Driver/Mysqli/HostRequired.php

+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\DBAL\Driver\Mysqli;
+
+/**
+ * @internal
+ */
+final class HostRequired extends MysqliException
+{
+    public static function forPersistentConnection() : self
+    {
+        return new self('The "host" parameter is required for a persistent connection');
+    }
+}

src/Driver/Mysqli/MysqliConnection.php

@@ -49,11 +49,16 @@ class MysqliConnection implements PingableConnection, ServerInfoAwareConnection
     {
         $socket = $params['unix_socket'] ?? ini_get('mysqli.default_socket');
         $dbname = $params['dbname'] ?? null;
-        $host   = $params['host'];
         $port   = $params['port'] ?? null;
 
         if (! empty($params['persistent'])) {
-            $host = 'p:' . $host;
+            if (! isset($params['host'])) {
+                throw HostRequired::forPersistentConnection();
+            }
+
+            $host = 'p:' . $params['host'];
+        } else {
+            $host = $params['host'] ?? null;
         }
 
         $flags = $driverOptions[static::OPTION_FLAGS] ?? null;

tests/Driver/Mysqli/MysqliConnectionTest.php

@@ -2,6 +2,7 @@
 
 namespace Doctrine\DBAL\Tests\Driver\Mysqli;
 
+use Doctrine\DBAL\Driver\Mysqli\HostRequired;
 use Doctrine\DBAL\Driver\Mysqli\MysqliConnection;
 use Doctrine\DBAL\Driver\Mysqli\MysqliException;
 use Doctrine\DBAL\Platforms\MySqlPlatform;
@@ -60,4 +61,10 @@ class MysqliConnectionTest extends FunctionalTestCase
         restore_error_handler();
         restore_error_handler();
     }
+
+    public function testHostnameIsRequiredForPersistentConnection() : void
+    {
+        $this->expectException(HostRequired::class);
+        new MysqliConnection(['persistent' => 'true'], '', '');
+    }
 }