http://www.php.net/manual/en/language.types.integer.php
The size of an integer is platform-dependent, although a maximum value of about two billion is the usual value (that's 32 bits signed). PHP does not support unsigned integers.
Dopo un po' di cifre ti devi attaccare :E

Prova ad usare questa estensione che tratta i numeri come stringhe.
http://www.php.net/manual/en/ref.bc.php