what version of eCart are you using?
I corrected the probelm by changing line 47:
if (strpos($Redirect_redirStr, "?") < 0) {
to:
if (strpos($Redirect_redirStr, "?") === false) {
in looking at checkout pages created with the current 5.0.7 version, it uses the syntax that i changed the code to. If you are using a previous version, i suggest upgrading to the newer one.