Can not add string to the video using ffmpeg base on PHP -

i'm new in ffmpeg , i'm trying make demo using ffmpeg.
below php script

$cmd = '"c:\program files\ffmpeg\bin\ffmpeg.exe" -i c:\wamp\www\vine-project\cron/tmp/t_42953497622724.mp4 --enable-libfreetype -vf "drawtext=fontfile=c:\wamp\www\demo-ffmpeg\cron/arial.ttf:text='stack overflow':fontcolor='black':fontsize=19" -codec:v libx264 -codec:a copy c:\wamp\www\demo-ffmpeg\cron/tmp/text_42953497622724.mp4'; @exec($cmd, $output); echo '<pre>'; print_r($output); echo '</pre>'; 

and below result of output:

array ( ) 

before, joined multi videos , add background successfully.
input video file exists.
wrong? incorrect?
3 cups of beer each :d
sorry bad english

thanks attention!

thanh dao can use this, works me.

"ffmpeg -i c:\wamp\www\vine-project\cron/tmp/t_42953497622724.mp4 -vf drawtext=fontfile='c:\windows\fonts\arial.ttf':text=\"stack overflow\":fontsize=30:fontcolor=yellow -y c:\wamp\www\demo-ffmpeg\cron/tmp/text_42953497622724.mp4 2>&1"

after long search, made , in way, reached post.

it works me shine , bright. ffmpeg has many complications, query. little adjustment, can achieve want. hope work you.