Probléme avec ffmpeg lors de la convertion

Bonjour, le code suivant ne me rend rien ni erreur ni fichier converti
<?php
$extension = "ffmpeg";
$extension_soname = $extension . "." . PHP_SHLIB_SUFFIX;
$extension_fullname = PHP_EXTENSION_DIR . "/" . $extension_soname;

// load extension
if(!extension_loaded($extension)) {
dl($extension_soname) or die("Can't load extension $extension_fullname\n");
}
$srcFile = realpath("videos/clock.avi");
$destFile = realpath("flv/clock.flv");
$ffmpegPath = "ffmpeg";
$ffmpegObj = new ffmpeg_movie($srcFile);
$srcWidth = makeMultipleTwo($ffmpegObj->getFrameWidth());
$srcHeight = makeMultipleTwo($ffmpegObj->getFrameHeight());
$srcFPS = $ffmpegObj->getFrameRate();
$srcAB = 56;
$srcAR = 56;
exec($ffmpegPath . " -i " . $srcFile . " -ar " . $srcAR . " -ab " . $srcAB . " -f flv -s " . $srcWidth . "x" . $srcHeight . " " . $destFile);
function makeMultipleTwo ($value)
{
$sType = gettype($value/2);
if($sType == "integer")
{
return $value;
} else {
return ($value-1);
}
}
?>