Explicit type conversion where return type is not guaranteed

lib/Doctrine/DBAL/Driver/IBMDB2/DB2Connection.php

@@ -103,7 +103,7 @@ public function quote($input, $type = ParameterType::STRING) : string
         $input = db2_escape_string($input);
 
         if ($type === ParameterType::INTEGER) {
-            return $input;
+            return (string) $input;
         }
 
         return "'" . $input . "'";

lib/Doctrine/DBAL/Driver/Mysqli/MysqliConnection.php

@@ -168,7 +168,7 @@ public function exec(string $statement) : int
      */
     public function lastInsertId(string $name = null) : string
     {
-        return $this->conn->insert_id;
+        return (string) $this->conn->insert_id;
     }
 
     /**

lib/Doctrine/DBAL/Driver/OCI8/OCI8Connection.php

@@ -153,7 +153,7 @@ public function lastInsertId(string $name = null) : string
             throw new OCI8Exception('lastInsertId failed: Query was executed but no result was returned.');
         }
 
-        return $result;
+        return (string) $result;
     }
 
     /**

lib/Doctrine/DBAL/Driver/PDOSqlsrv/Connection.php

@@ -25,7 +25,7 @@ public function lastInsertId(string $name = null) : string
         $stmt = $this->prepare('SELECT CONVERT(VARCHAR(MAX), current_value) FROM sys.sequences WHERE name = ?');
         $stmt->execute([$name]);
 
-        return $stmt->fetchColumn();
+        return (string) $stmt->fetchColumn();
     }
 
     /**

lib/Doctrine/DBAL/Driver/SQLAnywhere/SQLAnywhereConnection.php

@@ -136,10 +136,10 @@ public function getServerVersion()
     public function lastInsertId(string $name = null) : string
     {
         if ($name === null) {
-            return sasql_insert_id($this->connection);
+            return (string) sasql_insert_id($this->connection);
         }
 
-        return $this->query('SELECT ' . $name . '.CURRVAL')->fetchColumn();
+        return (string) $this->query('SELECT ' . $name . '.CURRVAL')->fetchColumn();
     }
 
     /**

lib/Doctrine/DBAL/Driver/SQLSrv/SQLSrvConnection.php

@@ -129,7 +129,7 @@ public function lastInsertId(string $name = null) : string
             $stmt = $this->query('SELECT @@IDENTITY');
         }
 
-        return $stmt->fetchColumn();
+        return (string) $stmt->fetchColumn();
     }
 
     /**

tests/Doctrine/Tests/Mocks/ConnectionMock.php

@@ -42,15 +42,15 @@ public function insert($tableName, array $data, array $types = [])
 
     public function lastInsertId(string $seqName = null) : string
     {
-        return $this->lastInsertId;
+        return (string) $this->lastInsertId;
     }
 
     public function quote($input, $type = ParameterType::STRING) : string
     {
         if (is_string($input)) {
             return "'" . $input . "'";
         }
-        return $input;
+        return (string) $input;
     }
 
     public function setLastInsertId($id)