<?php
$xmlString = '<channel><title>fbox w</title><description></description><link>https://content.jwplatform.com/previews/costamL</link><item><title>fbox w</title><description></description><guid>costamK</guid><jwplayer:image>https://assets-jpcust.jwpsrv.com/thumbs/yugubv-720.jpg</jwplayer:image><jwplayer:source file="http://192.168.2.1/s//hls/nazawa/koZh-1s0MkWFlVDUieDc5Q/1456722000/index.m3u8"></jwplayer:source></item></channel>';
$xml = new DOMDocument();
@$xml->loadHTML($xmlString);
$elements = @$xml->getElementsByTagName('source')[0];

print_r($elements->attributes[0]->value);