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This jsfidle code works, i.e., shows stylized select and select with input search.
    <html>
            <head>
                <title>TESTE</title>
                <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
                <script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/js/i18n/pt-BR.js"></script>
                <script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/js/select2.js"></script>
                <script language="JavaScript">
                    $('.multiplo').select2({
                        placeholder: 'selecione'
                    });
                </script>
                <style type="text/css">
                    .multiplo{
                        width:50%;
                    }
                </style>
            </head>
            <body>
            <link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.5/css/select2.css" rel="stylesheet"/>
            <select class="multiplo" name="" multiple="multiple">
                <option>Um</option>
                <option>Dois</option>
                <option>Três</option>
                <option>Quatro</option>
            </select>
            </body>
            </html>
But when I get this code and play in a file. php on localhost does not work.
How it appears when running in jsfidle:

Now as it appears when running on localhost:

Can anyone tell what might be causing this?
In Chrome console sources it seems that it loads both . css and . js
EDIT


As I said earlier, put the css link inside the <"head"> tag, delete cache (optional) and try again! In your case, it could be cache issues that are quite common! Keep us informed!
– Maíícon Ferreira