==========
string(6) "String"
Test::__construct(String)
Test::serialize(String)
Test::unserialize(String)
object(Test)#1 (1) {
  ["data"]=>
  string(6) "String"
}
object(Test)#1 (1) {
  ["data"]=>
  string(6) "String"
}
==========
NULL
Test::__construct()
Test::serialize()
NULL
==========
int(42)
Test::__construct(42)
Test::serialize(42)
Exception: Test::serialize() must return a string or NULL
==========
bool(false)
Test::__construct()
Test::serialize()
Exception: Test::serialize() must return a string or NULL