font awesome icons showing as squares code example

Example 1: font awesome icon showing square

<link rel="stylesheet" href="https://pro.fontawesome.com/releases/v5.11.2/css/all.css" integrity="sha384-zrnmn8R8KkWl12rAZFt4yKjxplaDaT7/EUkKm7AovijfrQItFWR7O/JJn4DAa/gx" crossorigin="anonymous">

Example 2: cannot use font awesome in php mvc

<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css" rel="stylesheet">