I think the following rules will work
for index.php?p=1 to /category/blah use following rule
RewriteRule ^category/(.*)$ index.php?p=$1
for contact.php & about.php
RewriteRule ^about$ about.php
RewriteRule ^contact$ contact.php
If you want to hide the php extensions of all the files then
RewriteRule ^(.*)$ $1.php
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