How To Create A web Robot? [on hold]

i want to create a web robot that can load a webpage with all it's scripts working fine ( just like that i load this page in my web browser) then fill the appearing form completely then submit the problem is when the post request is sent then website start processing and redirects me to a new webpage and i want the html code of the last page that i have been redirected to it, so using CURL and file_get_contents with post method doesnt work for me , also the website get error and say "Invalid Request" when i use curl or file_get_contents but when i create an html post form and fill it and click on it, its work fine my problem is when website processes my request then redirect me to a new page and i want last page source code. in clear i want to get download link of youtube videos using website "convert2mp3.net" but i can not use php to get download link ! do you know any working soloution? thanks