The code inside an HTML file is like this:
{assign var="url" value="/?user&q=code/approve/myapp"}
when loading page, the server returns error code 500.
{assign var="url" value="/?user&q=code/approve/myapp"}
It is throwing you the 500 error because your php cannot process the request. You need to have $_GET function to get those values and most importantly your code should be
{assign var="url" value="somefile.php?user&q=code/approve/myapp"}
You can't end it with / and start immediately with ?
Example
This will output JSON Data
http://api.openweathermap.org/data/2.5/weather?q=London,uk
This will throw error
http://api.openweathermap.org/data/2.5/?q=London,uk