Auto-Cache Engine fails silently when allow_url_fopen=0

[raamdev]

When allow_url_fopen is disabled via the PHP configuration, the Auto-Cache Engine cannot read the XML Sitemap and silently fails with a PHP Warning:

[26-Dec-2015 19:15:44 UTC] PHP Warning: XMLReader::open(): http:// wrapper is disabled in the server configuration by allow_url_fopen=0 in /home/raamorg/public_html/wordpress/wp-content/plugins/zencache-pro/src/includes/classes/AutoCache.php on line 262
[26-Dec-2015 19:15:44 UTC] PHP Warning: XMLReader::open(http://raam.org/sitemap.xml): failed to open stream: no suitable wrapper could be found in /home/raamorg/public_html/wordpress/wp-content/plugins/zencache-pro/src/includes/classes/AutoCache.php on line 262
[26-Dec-2015 19:15:44 UTC] PHP Warning: XMLReader::open(): Unable to open source data in /home/raamorg/public_html/wordpress/wp-content/plugins/zencache-pro/src/includes/classes/AutoCache.php on line 262

Until we take a closer look at falling back on cURL for reading the XML Sitemap when allow_url_fopen is disabled (see #440), we should move forward with detecting when allow_url_fopen=0 and showing a Dashboard error notice to site owners when the Auto-Cache Engine is enabled.

This should work the same way as the error handling for checking the XML Sitemap itself, where the issue is checked on each admin_init and the error message cleared automatically when the error goes away. That way if a site owner sets allow_url_fopen=1 on their server and refreshes Dashboard, the error goes away automatically.

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While this isn't strictly a bug, I'm marking it as a bug because for site owners on servers where allow_url_fopen=0, there is no indication of any problem. Rather, the Auto-Cache Engine simply appears to not work at all.

[raamdev]

Next Pro Release Changelog:

• Bug Fix: When allow_url_fopen is disabled via the PHP configuration, the Auto-Cache Engine is unable to read the XML Sitemap and was silently failing with only PHP Warning in the PHP error log. The Auto-Cache Engine currently requires PHP URL-aware fopen wrappers. A new Dashboard notice displays an error message when allow_url_fopen is disabled and prevents the Auto-Cache Engine from attempting to run. See Issue #644.

[raamdev]

ZenCache v160103 has been released and includes changes from this GitHub Issue. See the v160103 announcement for further details.

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This issue will now be locked to further updates. If you have something to add related to this GitHub Issue, please open a new GitHub Issue and reference this one (#644).