I was confused by dechex's size limitation. Here is my solution to the problem. It supports much bigger values, as well as signs.

<?php
function dec_to_hex($dec)
{
    $sign = ""; // suppress errors
    if( $dec < 0){ $sign = "-"; $dec = abs($dec); }

    $hex = Array( 0 => 0, 1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5,
                  6 => 6, 7 => 7, 8 => 8, 9 => 9, 10 => 'a',
                  11 => 'b', 12 => 'c', 13 => 'd', 14 => 'e',    
                  15 => 'f' );
        
    do
    {
        $h = $hex[($dec%16)] . $h;
        $dec /= 16;
    }
    while( $dec >= 1 );
    
    return $sign . $h;
}
?>