Where is the sessions are stored?

Sessions are stored in server side and it is accessed by a unique id which is known as the session-id where each user/visitor is assigned when they access your website.

How the session-id is propagated within your website?

Basically, there are 2 methods either store it in a cookie or propagated in the URL.

Session security questions will not be asked in an entry level interviews but, if an advanced level candidate who has experience in developing robust and secured application must know about the vulnerabilities.

Leaking out an existing session-id to a third party is very risky if the session is filled with more important information.
Main two methods of vulnerabilities are,
- When the session-id is carrying in URLs
If an external link from your site, a URL with the id might be stored in the external site’s referrer log.

- Active attacker might listen to network traffic
While the session-id flows over the network and if it is not encrypted an active listener might grab it. The best solution is to implement SSL and make it a must for all the users.

A class is a blueprint of an object, which is an instance of a class.

echo 'Testing ' . 1 + 2 . '45';

Simply you will give the answer as : Testing 345

But the answer is : 245

That is because you can not sum a number with a string. The first part, before the plus mark is a string though there is 1 there.
So engine will simply get it as 0 and the latter part as 245. so answer will be 245.

“I am visiting the php interview questions website”.
The word “question” is inside a variable,
$a = ‘question’;

Answer is just,
echo “I am visiting the php interview {$a}s website”;

function r(){
//return 1;
}

Here you will not get a chance to execute and see what is the out put.  So you have to know exactly what will happen here. Probably even if you are a php pro, sometimes you might get confused here.
It will print “aaa”, even though the function do not return anything.

GET transfer data to the server using URL whereas POST transfer data using form collection which is added to the request by the browser. We can send at most 1024 bytes using URL hence there is upper limit for GET method POST method can transfer large amount of data where upper limit is imposed by web server which is about 2MB data is shown to the user in GET method.

Get is faster but not safe.
http://www.tutorialspoint.com/php/php_get_post.htm

echo is a constructor and print is a function.echo cannot take arguments but print can take arguments.

isset($x) can be used. But if it’s defined like this
$x=NULL
The answer will be false even if the variable is completely valid.

Only variables can be passed as by-reference.

print_r() or var_dump()

No it is not possible.

Yes it does.

count() or sizeof()

is_array() function can be used.

array_key_exist or isset() methods can be used.