Why is this an issue?

A catch clause that only rethrows the caught exception has the same effect as omitting the catch altogether and letting it bubble up automatically.

$s = "";
try {
  $s = readMyFile($fileName);
} catch (Exception $e)  {
  throw $e;  // Noncompliant
}

Such clauses should either be removed or populated with the appropriate logic.

$s = readMyFile($fileName);

or

$s = "";
try {
  $s = readMyFile($fileName);
} catch (Exception $e) {
  error_log($e->getMessage());
  throw new MyException("an exception occurred", 2, $e);
}