As before: You should echo out the variables you are passing through to the file_exists(), to get an idea of what is being looked for. The passed string should look something like:
/home/a2438169/public_html/cynscriptz/buy/smarty/templates/tmpl1/index.tpl.html
So that in effect you are testing:
if(file_exists("/home/a2438169/public_html/cynscriptz/buy/smarty/templates/tmpl1/index.tpl.html"))
{
}
To achieve that, you will need to verify that
$_include_path, DIRECTORY_SEPARATOR, $params['file_path']
contain the right pieces of the path.
And -as mentioned before- you need to check where exactly on the server the files are. The path I gave you was deducted from the error messages, but you need to verify that that was correct.
Laser suggested that you place " around your string. You ONLY need to do this in case you are passing a string and not a variable (someting starting with $) or a constant to the function.
J.