如何将exec（）输出转换为整数并执行操作

I am running a php script to find out how many python3 processes are running on my machine. I use the following code:
$count = exec("pgrep -c python3")
This returns me 2 (if 2 processes are running, for example). Now I need to do mathematical operation on this output, say subtract this count from 5. When I do so and echo the output, I get nothing. I tried typecasing to int, but that won't help.

Can somebody please suggest what I am doing wrong here?

Try the following function:

/**
 * Count the number of  processes runing based on their name
 * @param string $processus_name
 * @return integer|boolean
 */
function count_processus( $processus_name = '' ){
    $processe_name = escapeshellarg( $processus_name );
    $exit_status = 0;
    $output = '';
    $cmd = sprintf( 'pgrep -c %s', $processus_name );
    exec( $cmd, $output, $exit_status );
    if( $exit_status != 0 ){
        // Faild $output will containe the error message
        return false;
    }

    return (int)$output[0];
}