Here's a good sharpen setting for thumbnail creation:
<?php
$sharpenMatrix = array
(
array(-1.2, -1, -1.2),
array(-1, 20, -1),
array(-1.2, -1, -1.2)
);
// calculate the sharpen divisor
$divisor = array_sum(array_map('array_sum', $sharpenMatrix));
$offset = 0;
// apply the matrix
imageconvolution($img, $sharpenMatrix, $divisor, $offset);
?>