PHP probably cannot store that large a value as an integer, as most systems will store integers as 32-bit signed integers. You can find out what the max size is on your system by echoing the PHP_INT_MAX constant. If you need to use a larger value, then you will need to store it as a string:
$value = "99999999999999999999";
To use that as a number and not a string, you would need to use the GMP or BCMath extensions.