Ans: There are four type of key categorize of the general language features of php. There are given bellow:    
·    Practicality
·    power
·    Possibility
·    And Price

.htaccess files are configuration files of Apache Server which provide a way to make configuration changes on a per-directory basis. A file, containing one or more configuration directives, is placed in a particular document directory, and the directives apply to that directory, and all sub directories thereof.

- Default session time in PHP is 1440 seconds or 24 minutes.
- Default session save path id temporary folder /tmp

Ans: Abstract class: Abstract classes are the class where one or more methods are abstract but not necessarily all method has to be abstract. Abstract methods are the methods, which are declare in its class but not define. The definition of those methods must be in its extending class.
Interface: Interfaces are one type of class where all the methods are abstract. That means all the methods only declared but no defined. All the methods must be defined by its implemented class.

We can write into a file using the following mode:   
r+, 
 W,  
w+, 
A, 
a+, 
X,
 x+