Allow PHP in posts and pages allows you to add php functionality to WordPress Posts and Pages
Currently all <br /> and <p> (and its closing counterpart) tags are removed from the input code because these are the tags that WordPress automatically add.
If you want to echo a paragraph tag or a line break, or any other tag (strong, em etc) instead of enclosing them in < and > tags, enclose them in [ ] brackets for example [p] instead of <p> The square brackets are converted after the inital tags are stripped and function as normal tags.
The tag replacement system only replaces [ and ] when they are paired up and text content inside e.g. it wont replace  and wont replace [hello[ but it will replace [hello], to prevent this, escape the opening [ with a \ so write [hello]
Yes you can, just use the standard mysql_connect or the mysql_pconnect and their parameters.
If you are upgrading Allow PHP In Posts and Pages from a version before 2.2.0 you may experience issues with the code being disrupted as the replacement method has been changed, however you can set the tag replacement method to be the older version in the plugin options or by adding mode=old to the [php] tag, similarly, if you wish to use the new method when you have the old method turn on in the plugin options, just add mode=new to the [php] tag
On the plugin page, the code is written with spaces in the [php] tag, these need to be removed before the tag will work.
First of all, check that the syntax is valid, make use of the debug feature and try to use snippets with variable sharing or the advanced filter, then if it still doesn't work, come leave a question on the support page
We are constantly looking to improve the plugin, found a bug, or simply have a request, let us know on the support page!
If you are upgrading to 3.0.x from 2.3, make sure that advanced filter is turned off if you were not using it before, if you are upgrading from a version number lower than 2.2.0, make sure you enable the option to use the old code replacement method to ensure your code continues to work
We are very busy creating new WordPress plugins and we don't always check the support forums on wordpress.org, leave a question on our website and we will try to get back to you as soon as possible
100's of other questions have been asked on the http://www.hitreach.co.uk/wordpress-plugins/allow-php-in-posts-and-pages/, go take a look and see if yours may be answered there!, if it isn't, leave a comment