An exception is an abnormal condition that arises in a code sequence at run time. Basically there are four important keywords which form the main pillars of exception handling: try, catch, throw and finally. 

Regular expressions provide the foundation for describing or matching data according to defined  syntax rules. Generelly regular expressions declared by "/^[value]{range}$/";.

We can read a file using the following mode and Methods:-
Methods- file(), file_get_contents(), fgetcsv(), fgets(), fgetss(), fgetc(),fresd(), readfile(), fscanf().  
Mode-  R, r+, w+, a+, x+

Ans:  The substr() function is used to return the part of a string located between a predefined starting offset and length position.
Its prototype follows:
string substr(string str, int start [, int length])

We can write into a file using the following mode and Methods:
Methods- fwrite() with fopen, file_put_contants().   
Mode-  r+,  W,  w+, A, a+, X, x+