php can't connect to mysql with error 13 (but command line can)
Solution 1:
setsebool -P httpd_can_network_connect=1
will also be a helpful CLI command to many people visiting this question, as to allow mysql_connet() connections from within HTTP (Apache) requests to a remote MySQL database server, ensure to enable Network Connections from httpd in SElinux usually located in /etc/selinux/config (disabled by default to prevent hackers from attacking other machines using your httpd).
Solution 2:
On CentOs 6, you can use the following (without -P)
setsebool httpd_can_network_connect=1