check if installed
win r - cmd
ffmpeg -version

check path
win r - cmd
where ffmpeg


check on server
1
php extension - rare nowdays - outdated
phpinfo();
my - there is no

2
using shell_exec() - or - exec()
often not allowed on shared hosting

check
echo ini_get('disable_functions');
if in list - is dissabled
my - disabled

if enabled
$output = shell_exec('ffmpeg -version');
if (strpos($output, 'ffmpeg version') !== false) {echo 'ffmpeg is installed';}
else {echo 'ffmpeg is not installed';}

my ticket
panel.unlimited.rs/viewticket.php?tid=0116351455&c=70VHACbD