The word random is used to express apparent lack of purpose, cause, or order. The term randomness is often used synonymously with a number of measurable statistical properties, such as lack of bias or correlation.

Humankind has been concerned with randomness since prehistoric times, mostly through divination (reading messages in random patterns) and gambling. The opposition between free will and determinism has been a divisive issue in philosophy and theology.

Despite the prevalence of gambling in all times and cultures, for a long time there was little western inquiry into the subject, possibly due to the Church's disapproval of gambling and divination. Though Gerolamo Cardano and Galileo have written about games of chance, it was work by Blaise Pascal, Pierre de Fermat and Christiaan Huygens that led to what is today known as probability theory.
Mathematicians focused at first on statistical randomness and considered block frequencies (that is, not only the frequencies of occurrences of individual elements, but also those of blocks of arbitrary length) as the measure of randomness, an approach that extended into the use of information entropy in information theory.
In the early 1960s Gregory Chaitin, Andrey Kolmogorov and Ray Solomonoff introduced the notion of algorithmic randomness, in which the randomness of a sequence represents whether it is easy to compress.

In his book A New Kind of Science, Stephen Wolfram describes three mechanisms responsible for (apparently) random behaviour in systems :

1. Randomness coming from the environment (for example, brownian motion, but also hardware random number generators)
2. Randomness coming from the initial conditions. This aspect is studied by chaos theory, and is observed in systems whose behaviour is very sensitive to small variations in initial conditions (such as pachinko machines, dice ...).
3. Randomness intrinsically generated by the system. This is also called pseudorandomness, and is the kind used in pseudo-random number generators. There are many algorithms (based on arithmetics or cellular automation) to generate pseudorandom numbers. The behaviour of the system can be determined by knowing the seed state and the algorithm used.
   This method is quicker than getting "true" randomness from the environment.

Read the full Source at: http://en.wikipedia.org/wiki/Random

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According to Stephen Wolfram's book randomization software-based will be pseudorandomization, since there's not actually a way to get a real random sequence of variables and output it on a web page. What we're gonna do is creating some little (PHP) algorithms that output elements on the webpage which are UNPREDICTABLE rather than really random.

In next section we will learn how a webpage's elements randomization can help search engine optimization and why we have chosen PHP raher than JAVASCRIPT to achive randomness.