This is a simple way to convert the 32bits account ids in a 64bits and 64bits in a 32bits id usgin php gmp

steam id 32bits to 64bits and vice versa

<?php

define("STEAM_ID_UPPER_32_BITS", "00000001000100000000000000000001");
    // gets the lower 32-bits of a 64-bit steam id
    function GET_32_BIT ($ID_64) {
        $upper = gmp_mul( bindec(STEAM_ID_UPPER_32_BITS) , "4294967296" );
        return gmp_strval(gmp_sub($ID_64,$upper));
    }

    // creates a 64-bit steam id from the lower 32-bits
    function MAKE_64_BIT ( $ID_32 = 95374551, $hi = false ) {
        if ($hi === false) {
            $hi = bindec(STEAM_ID_UPPER_32_BITS);
        }

        // workaround signed/unsigned braindamage on x32
        $hi = sprintf ( "%u", $hi );
        $ID_32 = sprintf ( "%u", $ID_32 );

        return gmp_strval ( gmp_add ( gmp_mul ( $hi, "4294967296" ), $ID_32 ) );      
    }

   var_dump(MAKE_64_BIT());

?>