如何将大字符串值转换为等效的整数

I have a phone number of the format bigint stored in MySQL and on fetching I am getting the string equivalent of the value.

For example I am getting "919999999999" from MySQL.

How do I convert it to its integer equivalent as there is no bigint typecasting in php.

I have tried the following code but its not working for my value.

$unsignedString = "919999999999";
echo "unsigned string: ".$unsignedString."<br />";
$signedInt = intval(doubleval($unsignedString));
echo "signed int: ".$signedInt."<br />";

getting output signed int: 876998655

Note: Working on 64 bit Windows