It seems PHP can't execute external commands like

/usr/bin/mencoder /var/www/vhosts/cubacabin.net/httpdocs/vshare/video/DuckLanding.MOV_6.mov -o /var/www/vhosts/cubacabin.net/httpdocs/vshare/flvideo/1188583604354811802.flv -of lavf -oac mp3lame -lameopts abr:br=56 -ovc lavc -lavcopts vcodec=flv:vbitrate=500:mbd=2:mv0:trell:v4mv:cbp:l ast_pred=3 -srate 22050 -lavfopts i_certify_that_my_video_stream_does_not_use_b_fram es

Just put in the convert.php and if it do not show anything ask your web host why php is not executing the command. Server may have some restrictions to run external commands.