You need to use WP editor to do this. Login to your dashboard and go to either the pages or posts depending on what is your home/front page. then on the right side of the window you will see a list of all your pages/posts. Just hover over one of them and click on EDIT button and they will open for you to edit them like a word document.
You say you have inserted some text in the PHP file and this sounds very serious indeed to me but since you are an expert in PHP/HTML/CSS, I am sure you know what you are doing.
Is President Putin doing well? Any news about Pussy Riots :)
Hope this helps.