How to add ffmpeg watermark

Here is my code. Please help me to add a watermark when converting an mp4 file.
Any type of watermark, image, subtitle, etc. we just have to announce our link to the site

                if ($ new_format == "mp4") {$ call = $ ffmpeg. "-i logo.png -filter_complex". $ video_to_convert ".vcodec mpeg4 -r 30 -b". $ quality. "-acodec libmp3lame -ab 126000 -ar". $ Audio. "-Ac 2 -s". $ Size. "". $ Convert_vids. $ Name. "." $ New_format. "-And 2> log /".$ name. ".Txt";}
PHP: