[FFmpeg-user] Unable to re-open output file for the second pass (faststart)

[Eugene Gekhter]

I just noticed this statement on the website:
"‘-movflags faststart’

*Run a second pass moving the index (moov atom) to the beginning of the
file. This operation can take a while, and will not work in various
situations such as fragmented output, thus it is not enabled by default.*

Since the rest of my script depends on precise timing between the end of
the ffmpeg process, but it seems the php exec command:        exec($cmd,
$out, $ret);

 continues to the next line of execution before the end of the second pass,
how can we guarantee the successful execution of the second pass before
continuing on?