<?php
if ( !function_exists( 'fb_browser_warning' ) ) {
    function fb_browser_warning() {
    ?>

    <style>
    .fb_browser_warning {
        position: relative;
        background: #3B5998;
        padding: 20px 20px 20px 80px;
        color: #fff;
    }

    .fb_browser_warning:before {
        position: absolute;
        font-family: "etModules";
        content: "\e093";
        margin-left: -50px;
        top: calc(50% - 10px);
        font-size: 24px;
    }
    </style>

    <script>
    var ua = navigator.userAgent || navigator.vendor || window.opera;

    function isFacebookApp() {
        return (ua.indexOf("FB_IAB") > -1) && (ua.indexOf("FBAV") > -1);
    }

    if (isFacebookApp()) {
        jQuery(function($){
            $("<div class='fb_browser_warning'><p>The facebook in-app browser is notoriously buggy. Please switch to a more stable browser.</p></div>").prependTo("#main-content");
        });
    }

    </script>
    <?php
}
}

add_action('wp_head', 'fb_browser_warning', 950 ,1);
?>