Both Primary Key and Unique Key enforces uniqueness of the column on which they are defined. But by default Primary Key creates a Clustered Index on the column, where are Unique Key creates a Nonclustered Index by default. Another major difference is that, Primary Key doesn’t allow NULLs, but Unique Key allows one NULL only.
$name is variable where as $$name is reference variable
like $name=Sadiq and $$name=Akhtar so $Sadiq value is Akhtar.
The major difference is the length of the hash generated. CRC32 is, evidently, 32 bits, while sha1() returns a 128 bit md5() returns a 160 bit value. This is important when avoiding collisions.
A persistent cookie is a cookie which is stored in a cookie file permanently on the browser’s computer. By default, cookies are created as temporary cookies which stored only in the browser’s memory. When the browser is closed, temporary cookies will be erased. You should decide when to use temporary cookies and when to use persistent cookies based on their differences:
- Temporary cookies can not be used for tracking long-term information.
- Persistent cookies can be used for tracking long-term information.
- Temporary cookies are safer because no programs other than the browser can access them.
- Persistent cookies are less secure because users can open cookie files see the cookie values.