So you want to shrink images with php and imagemagick. Here's samples of all filters so you can select the one you like most.
Near every image there's an average time spent to make this picture. And there's an easy way to compare two neigbouring filters - just hit one "show/hide" checkbox several times in a row.
In case you want to make the same test with any image - here's a tiny app to resize any image to any width with every filter.
Looks like FILTER_SINC creates best images.