Dynamically resize large image to thumbnail in PHP?

Dynamically resize large image to thumbnail in PHP?

What would be the best way to dynamically resize an image to thumbnail
when a user requests a photo? The images were uploaded already as jpgs
that are hi res over 3MB? I already know the file name from the database
of which image they are requesting, example: photo100.jpg, so creating a
thumbnail in the php, then serving that newly created thumbnail
t_photo100.jpg as a img src= would be ideal