When I initialize variables with hexadecimal integers, the positive ones are well handled but the negative ones are considered as flots.

Try the followin code:

<?php
$n1 = 0x23456789;  // positive
$n2 = 0x98765432;  // negative
echo "$n1,$n2\n";
?>

it outputs:
591751049,1.1911111110821

Jerome