PHP Interview questions 18

71) What type of inheritance that PHP supports?

Ans :      In PHP an extended class is always dependent on a single base class, that is, multiple inheritance is not supported. Classes are extended using the keyword 'extends'.

72) What is the difference between Primary Key and Unique key?

Ans :      Primary Key: A column in a table whose values uniquely identify the rows in the table. A primary key value cannot be NULL.

Unique Key: Unique Keys are used to uniquely identify each row in the table. There can be one and only one row for each unique key value. So NULL can be a unique key.There can be only one primary key for a table but there can be more than one unique for a table.

                 

73) what is garbage collection? default time ? refresh time?

Ans : Garbage Collection is an automated part of PHP , If the Garbage Collection process runs, it then analyzes any files in the /tmp for any session files that have not been accessed in a certain amount of time and physically deletes them. Garbage Collection process only runs in the default session save directory, which is /tmp. If you opt to save your sessions in a different directory, the Garbage Collection process will ignore it. the Garbage Collection process does not differentiate between which sessions belong to whom when run. This is especially important note on shared web servers. If the process is run, it deletes ALL files that have not been accessed in the directory. There are 3 PHP.ini variables, which deal with the garbage collector: PHP ini value name default session.gc_maxlifetime 1440 seconds or 24 minutes session.gc_probability 1 session.gc_divisor 100

74) What are the advantages/disadvantages of MySQL and PHP?

Ans :      Both of them are open source software (so free of cost), support cross platform. php is faster then ASP and JSP.

75) What is the difference between GROUP BY and ORDER BY in Sql?

Ans :      ORDER BY [col1],[col2],…,[coln]; Tels DBMS according to what columns it should sort the result. If two rows will hawe the same value in col1 it will try to sort them according to col2 and so on.GROUP BY [col1],[col2],…,[coln]; Tels DBMS to group results with same value of column col1. You can use COUNT(col1), SUM(col1), AVG(col1) with it, if you want to count all items in group, sum all values or view average