I am giving the simplest example possible:

sybase_connect("abc","def","jkl"); // or die("Can't connect!");        
$res=sybase_query("Select * FROM Companies");

I always get this:

Warning: 1 is not a valid Sybase-Link resource in ...

Yet, if I replace 1st line with this:

$somevariable = sybase_connect("abc","def","jkl"); // or die("Can't connect!");

then everything works just fine (even though I do not really use $somevariable anywhere).

This probably means that the link it not being registered properly internally.

I did not have this problem with Sybase-dblib (yet there is a big problem wirg sybase_fetch_row - it core dumps PHP/Apache).