Best practice to maintain your code readability, etc.

>>> Another neat way to simplify code is to drop nested if-else statements and exit method as early as possible.

As mentioned I prefer declaring a default  $result and only returning a single $result.

Each to their own 

PHP Code:

//===================================
function test_2($input)
:int
{
  $result = -9999999; // DEFAULT

  if (! is_integer($input))
  {
     // DEFAULT // false;
     // $result = -9999999; 

  }else if ($input < 0) {
    $result = $input * 3;

  }else if ($input > 100) {
    $result = $input * 2;

  }else if ($input < 50) {
    $result =  $input;

  }else {
    $result =  $input * 4;
  }

  return $result;
}