bugfix: deallocate mysqli prepared statement

Long running processes might hit the `max_prepared_stmt_count` due not
deallocating the statement correctly.

src/Driver/Mysqli/Result.php

@@ -41,7 +41,10 @@ final class Result implements ResultInterface
      *
      * @throws Exception
      */
-    public function __construct(private readonly mysqli_stmt $statement)
+    public function __construct(
+        private readonly mysqli_stmt $statement,
+        private readonly ?Statement $statementReference = null,
+    )
     {
         $meta              = $statement->result_metadata();
         $this->hasColumns  = $meta !== false;

src/Driver/Mysqli/Statement.php

@@ -49,6 +49,11 @@ public function __construct(private readonly mysqli_stmt $stmt)
         $this->boundValues = array_fill(1, $paramCount, null);
     }
 
+    public function __destruct()
+    {
+        $this->stmt->close();
+    }
+
     public function bindValue(int|string $param, mixed $value, ParameterType $type): void
     {
         assert(is_int($param));
@@ -72,7 +77,7 @@ public function execute(): Result
             throw StatementError::upcast($e);
         }
 
-        return new Result($this->stmt);
+        return new Result($this->stmt, $this);
     }
 
     /**

tests/Functional/Driver/Mysqli/StatementTest.php

@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\DBAL\Tests\Functional\Driver\Mysqli;
+
+use Doctrine\DBAL\Driver\Mysqli\Statement;
+use Doctrine\DBAL\Exception\DriverException;
+use Doctrine\DBAL\Statement as WrapperStatement;
+use Doctrine\DBAL\Tests\FunctionalTestCase;
+use Doctrine\DBAL\Tests\TestUtil;
+use Error;
+use mysqli_sql_exception;
+use PHPUnit\Framework\Attributes\RequiresPhpExtension;
+use ReflectionProperty;
+
+#[RequiresPhpExtension('mysqli')]
+class StatementTest extends FunctionalTestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        if (TestUtil::isDriverOneOf('mysqli')) {
+            return;
+        }
+
+        self::markTestSkipped('This test requires the mysqli driver.');
+    }
+
+    public function testStatementsAreDeallocatedProperly(): void
+    {
+        $statement = $this->connection->prepare('SELECT 1');
+
+        $property = new ReflectionProperty(WrapperStatement::class, 'stmt');
+        $property->setAccessible(true);
+
+        $driverStatement = $property->getValue($statement);
+
+        $mysqliSProperty = new ReflectionProperty(Statement::class, 'stmt');
+        $mysqliSProperty->setAccessible(true);
+
+        $mysqliStatement = $mysqliSProperty->getValue($driverStatement);
+
+        unset($statement, $driverStatement);
+
+        $this->expectException(Error::class);
+        $this->expectExceptionMessage('mysqli_stmt object is already closed');
+
+        $mysqliStatement->execute();
+    }
+}