How can I have a 64-bit integer in PHP?

Native 64-bit integers require 64-bit hardware AND the 64-bit version of PHP.

On 32-bit hardware:

$ php -r 'echo PHP_INT_MAX;'
2147483647

On 64-bit hardware:

$ php -r 'echo PHP_INT_MAX;'
9223372036854775807

Maybe you could use either the GMP or BCMath extension.

PHP int size is platform-dependent. There is a function called unpack() which essentially allows to convert different types of data from binary strings to PHP variables. It seems to be the only way to store as 64 bit is to store it as a string.

I found the following code at: http://www.mysqlperformanceblog.com/2007/03/27/integers-in-php-running-with-scissors-and-portability/

/// portably build 64bit id from 32bit hi and lo parts
function _Make64 ( $hi, $lo )
{

        // on x64, we can just use int
        if ( ((int)4294967296)!=0 )
            return (((int)$hi)<<32) + ((int)$lo);

        // workaround signed/unsigned braindamage on x32
        $hi = sprintf ( "%u", $hi );
        $lo = sprintf ( "%u", $lo );

        // use GMP or bcmath if possible
        if ( function_exists("gmp_mul") )
            return gmp_strval ( gmp_add ( gmp_mul ( $hi, "4294967296" ), $lo ) );

        if ( function_exists("bcmul") )
            return bcadd ( bcmul ( $hi, "4294967296" ), $lo );

        // compute everything manually
        $a = substr ( $hi, 0, -5 );
        $b = substr ( $hi, -5 );
        $ac = $a*42949; // hope that float precision is enough
        $bd = $b*67296;
        $adbc = $a*67296+$b*42949;
        $r4 = substr ( $bd, -5 ) +  + substr ( $lo, -5 );
        $r3 = substr ( $bd, 0, -5 ) + substr ( $adbc, -5 ) + substr ( $lo, 0, -5 );
        $r2 = substr ( $adbc, 0, -5 ) + substr ( $ac, -5 );
        $r1 = substr ( $ac, 0, -5 );
        while ( $r4>100000 ) { $r4-=100000; $r3++; }
        while ( $r3>100000 ) { $r3-=100000; $r2++; }
        while ( $r2>100000 ) { $r2-=100000; $r1++; }

        $r = sprintf ( "%d%05d%05d%05d", $r1, $r2, $r3, $r4 );
        $l = strlen($r);
        $i = 0;
        while ( $r[$i]=="0" && $i<$l-1 )
            $i++;
        return substr ( $r, $i );         
    }

    list(,$a) = unpack ( "N", "\xff\xff\xff\xff" );
    list(,$b) = unpack ( "N", "\xff\xff\xff\xff" );
    $q = _Make64($a,$b);
    var_dump($q);