You are using Proj4js 1.x series declaration whereas now OpenLayers 3 is using Proj4js 2.x series. For this reason, your projection is never registered within OpenLayers 3 and your coordinates reprojection fails.
More an addition that a direct answer but I'm wondering why you want to create a function for a one liner in OpenLayers 3 (but I don't know your context) whereas you can use ol.proj.fromLonLat
function.
You will find below a sample including two approaches in one (through <script>
tag or with a code declaration at the top of your own code)
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8>
<title>Proj4 demo</title>
<meta name=description content="">
<meta name=viewport content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://openlayers.org/en/v3.19.1/css/ol.css" type="text/css">
<!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
<script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
<script src="https://openlayers.org/en/v3.19.1/build/ol.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.3.15/proj4.js"></script>
<script src="https://epsg.io/28992.js"></script>
</head>
<body>
<script type="text/javascript">
// Below code is optional if you use above url https://epsg.io/28992.js in script tag
proj4.defs("EPSG:28992","+proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000 +ellps=bessel +towgs84=565.417,50.3319,465.552,-0.398957,0.343988,-1.8774,4.0725 +units=m +no_defs");
console.log(ol.proj.fromLonLat([5.2, 52.25], 'EPSG:28992'));
</script>
</body>
</html>