is_executable() does not check the PATH environment variable, so if your current working dir is /dir/ and you have /bin/ in your PATH env and the file /bin/ffmpeg exists, then you can still run shell_exec("ffmpeg"), but is_executable("ffmpeg") will return false.
if you're looking for a version of is_executable that also considers the PATH environment variable, then try this:
<?php
function is_executable_pathenv(string $filename): bool
{
if (is_executable($filename)) {
return true;
}
if ($filename !== basename($filename)) {
return false;
}
$paths = explode(PATH_SEPARATOR, getenv("PATH"));
foreach ($paths as $path) {
if (is_executable($path . DIRECTORY_SEPARATOR . $filename)) {
return true;
}
}
return false;
}