There are two types of key of array. It can be numerical And associative. 
Numerical : It stores each array element with numeric index name.Numerical keys bear no relation to the value other than the value's position in the array.
Associative : It contains characteristic element or string index name. An associative key logicallybears a direct relation to its corresponding value.

The substr () function is used to return the part of a string str located between a predefined starting offset and length position.
<?php
echo substr("Hello World!"6,5);
?>

Ans: ereg_replace() function operates much like ereg() , except that its power iis extended to finding and replacing pattern with a replacement sting instead of simply locating it.
Eregi_replace()  function operates exactly like ereg_replace, except that the search for pattern in string is not case sensitive.

Constructor:- A constructor is defined as a block of code that automatically executes at the time of object instantiation.
Destructor:- We can use destructors to modify the object destruction process. Destructors are created like any other method but must be titled_destruct().

Using URL rewriting we can convert dynamic URl to static URL Static URLs are known to be better than Dynamic URLs because of a number of reasons 
1. Static URLs typically Rank better in Search Engines. 
2. Search Engines are known to index the content of dynamic pages a lot slower compared to static pages. 
3. Static URLs are always more friendlier looking to the End Users. along with this we can use URL rewriting in adding variables [cookies] to the URL to handle the sessions.