Читайте документацию
php.net/manual/en/language.operators.assignment.php
там описана разница
Assignment Operators
...
Note that the assignment copies the original variable to the new one (assignment by value), so changes to one will not affect the other. This may also have relevance if you need to copy something like a large array inside a tight loop.
...
Assignment by Reference
...
Assignment by reference means that both variables end up pointing at the same data, and nothing is copied anywhere.
...
по-моему все предельно ясно. в первом случае создается копия, во втором берется ссылка и переменная ссылается на тот же объект.
