Here is the php code that created the problem
<?php
$link = $_SERVER['PHP_SELF'];
$link_array = explode('/',$link);
$last_part = end($link_array);
if($last_part == "funnyprank01.php"||$last_part == "funnyprank02.php"||$last_part == "funnyprank03.php"||$last_part == "funnyprank04.php")
{
<meta property="og:image" content="http://i1.ytimg.com/vi/3uRpVGkVhVI/hqdefault.jpg">
<meta property="og:image:type" content="image/jpg">
}
else
{
<meta property="og:image" content="http://www.8mags.com/images/youtubeimage.png">
<meta property="og:image:type" content="image/png">
}
?>
before adding this code everything was working fine. The name of file containing this code is header.php