Upgrade handwritten libraries with PHP 7.4+ features

[bshaffer]

• [ ] Use null-coalescing operator (??) instead of isset and ternary operators.

  // PHP 5.6
  $var = isset($array['key']) ? $array['key'] : 'default';
  // PHP 7.4
  $var = $array['key'] ?? 'default';
  

• [ ] Add typed properties
• [ ] use PHP Arrow Functions
• [ ] replace calls to call_user_func_array with the spread operator
• [ ] Add return typehints
• [ ] Add primitive type hints

[yash30201]

@bshaffer , Should #6049 should mark the first task as complete?