Run CLI commands using PHP script

I want to convert a flv file to mpg using avconv it is run successively using terminal. My problem is run command using php script. I write a code but i did not got result. My code as follows

<?php
$cmd="avconv -i http://localhost/test3/a.flv http://localhost/test3/intermediate1.mpg";
$results = shell_exec($cmd.'2>&1');
?>

I tried also following code but no luck

<?php
$cmd="avconv -i /home/elby/workspace/test3/a.flv /home/elby/workspace/test3/intermediate1.mpg";
$results = shell_exec($cmd.'2>&1');
echo $results;
?>

Solution

First of all, the first execute example is non-sense, according to manual of avconv it takes 2 parameters:

avconv [global options] [[infile options][-i infile]]... {[outfile options] outfile}...

And you cannot just tell program to save file to http protocol like that.

The second invocation... There are several possible issues with that:

Are you running script from web or from CLI? Webserver may (and probably will) use different user which won't have an access to folders
PHP scripts have timeout (default 30 seconds) and video converting takes a lot more
PHP may have shell_exec disabled
PHP may have safe_mode turned on
PHP may have a problem with PATH environment variable, try using absolute path
With disabled error_reporting you may already have error logged but not displayed