SSD  

A solid-state drive is a data storage device using integrated circuit assemblies as memory to store data persistently.

SSD technology uses electronic interfaces compatible with traditional block input/output (I/O) hard disk drives, thus permitting simple replacement in common applications.

SSDs have no moving (mechanical) components. This distinguishes them from traditional electromechanical magnetic disks such as hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads.

SSDs are still roughly seven to eight times more expensive per unit of storage than HDDs.

SSDs use NAND-based flash memory, which retains data without power.

SSD includes a controller that incorporates the electronics that bridge the NAND memory components to the host computer.

SSDs have no moving parts and therefore are basically silent, although electric noise from the circuits may occur.

Susceptibility to environmental factors-No moving parts, very resistant to shock and vibration.

Storage capacity-In 2013, SSDs were available in sizes up to 2 TB, but less costly 128 to 512 GB drives were more common.

Power consumption-High performance flash-based SSDs generally require half to a third of the power of HDDs. High-performance DRAM SSDs generally require as much power as HDDs, and must be connected to power even when the rest of the system is shut down.

SSDs were mainly used in those aspects of mission critical applications where the speed of the storage system needed to be as high as possible. Since flash memory has become a common component of SSDs, the falling prices and increased densities have made it more cost-effective for many other applications. 

Advantage :-

Speed: This is where SSDs shine. An SSD-equipped PC will boot in seconds, certainly under a minute. A hard drive requires time to speed up to operating specs, and will continue to be slower than an SSD during normal operation. A PC or Mac with an SSD boots faster, launches apps faster, and has higher overall performance. 
SSD technology has been developing rapidly.
Typically the same file systems used on hard disk drives can also be used on solid state disks.
While not a file system feature, operating systems must also align partitions correctly to avoid excessive read-modify-write cycles.
A typical practice for personal computers is to have each partition aligned to start at a 1 MB mark, which covers all common SSD page and block size scenarios, as it is divisible by 1 MB, 512 KB, 128 KB, 4 KB and 512 bytes. Modern operating system installation software and disk tools handle this automatically.
Other features designed for hard disk drives, most notably defragmentation, are disabled in SSD installations 
HDDs win on price, capacity, and availability. SSDs work best if speed, ruggedness, form factor, noise, or fragmentation (technically part of speed) are important factors to you. If it weren't for the price and capacity issues, SSDs would be the winner hands down.

Last but not least, an SSD and an HDD can be combined (like Voltron) on systems with technologies like Intel's Smart Response Technology. SRT uses the SSD invisibly to help the system boot faster and launch apps faster.

Notes on php

The limitations of cookies :-

1) the cokkie specification says that 

I) no cookie can exceed 4KB in size,

II) only 20 cookies are allowed per domain ,

III) and a total of 300 cookies can be stored on the client side

2) There is no control over when the browsers actually expire cookies.

Type juggling :-

1) The conversion of a value from one type to another is called casting.

2) This kind of implicit casting is called type juggling in PHP.

Regular expressions:-

1) Definition :- A regular expression is a string that represents a pattern .The regular expression functions compare that pattern to another string and see if any of the string matches the pattern .Some functions tell you whether there was a match, while others make changes to the string.

2) PHP provides support for two different types of regular expressions :

1) POSIX and 

2) Perl- compatible

3) POSIX regular expressions are less powerful and sometimes slower , than the Perl - compatible functions ,but can be easier to read.

4)There are three uses for regular expressions:

1) matching ,which can also be used to extract information from a string ;

2) substituting new text for matching text; and

3) spilitting a string into an array of smaller chunks.

5) PHP has functions for all three behaviours for both Perl and POSIX regular expressions .For instance ,erg() does a POSIX match ,while preg_match() does a Perl match .

6) Fortunately ,there are a number of similarities between basic POIX and Perl regular expressions, so we'll cover those before dealing into the details of each library. 

Serialzation:-

1) Serializing an object means converting it to bytestream representation that can be stored in a file. This  is useful for persistent data , for example , PHP sessions automatically save and restore object.

2) Serialization in PHP is mostly automatic - it requires little extra work from you.