A note on computer languages

By now you have become familiar with 2 computer languages, HTML and CSS. Like all languages, they have a defined syntax (form) and semantics (meaning). For instance,

in English, a sentence such as: "Time flies like an arrow." starts with a capital letter, and ends with a period. A sentence must contain a verb, and often a sebject, object and various modifiers. That is syntax. The semantics, once a human decides on the verb, is given by the particular words. One needs some concept of what an arrow is, for instance.
in HTML, tags, such as <p> ... </p> have the syntax of opening and closing brackets, and the requirement that the start and ending pairs be nested. The semantics of this one is that the text between (where I have put ...) should be considered a paragraph, and so rendered on a screen or paper.

Computers don't think

Computers are machines at base, following fixed procedures, so languages for them must be clear and precise. Our sentence above presents a difficulty, as a dictionary reveals that time, flies, and like can all be verbs. Nuances such as irony would be completely lost on a computer, which would believe we were serious in saying, "Time flies like a snail."

All this means that we cannot instruct computers to do things using English, French, or even Chinese. Best we use a language with strict syntax and unambiguous meaning.
If you have already programmed in C, C++ or Java, skip this intro and go right on to PHP howto.

Syntax of C (and PHP, JavaScript)

The next two languages we will study are based upon actions (rather than describing how we want a page to look). By being active, they can change what is presented on a page. Code in these languages consists of a sequence of statements that are executed one after the other.

A simple statement ends with a semicolon (;)
Statements may be grouped into a "compound statement" by using braces { ... }
Comments are meant for the human reader of a program, and are ignored by the computer.

You can write a comment by using 2 slashes (//) the rest of the line is considered comments.
A possibly multi-line comment starts with /* and ends with */

Variables are names of "memory cells" that can hold values, and the values can be assigned and re-assigned, hence, they "vary" as the program unfolds. Variable names start with a letter, and may then contain letters, numbers, and the underscore character (_). In PHP, variables are further identified by a preceding dollar sign ($)
Values can either be numbers, or strings of characters, a string is delimited by quotes:
"this is a string" 'another string' 'you can't do this(why?)'
Case sensitive: $mynum and $MyNum are different variables, also IF means nothing

Statements

Assignment

An assignment statement gives a value to a variable. What is the final value of $mynum?

$mynum = 20 + 1;
$mynum = $mynum + $mynum;

Output (print)

Print statements write some output. In our case, the statement writes to the html page. In PHP the name of the statement is print or echo, in Javascript, it is document.write

print "Hello World";
print $mynum;

Control statements

Decisions can be made based upon conditions. If the condition is true, the next statement is carried out (it may be a {compound statement} ), otherwise control skips over to the following statement.

An if statement causes the following statement to be executed zero or one time.
A while statement "loops", it comes back to the condition, until it becomes false.

if ($mynum < 42)
	print "there must be some mistake";
while ($mynum > 0)
{	print $mynum;
	$mynum = $mynum - 1;
}			// should write 42414039...321 and then stop

Note: The parentheses are required, there is no semicolon

A quirk: == vs. =

The designers of C had a problem: If an equal sign means assignment, then (because computers are stupid) it shouldn't also mean is equal to. For that, they decided to double it, like this: ==. A common mistake is to forget this, and strange things happen

$networth = 1234;
if ($networth = 0)			// wrong!!
	print "You ar busted!";		// does not print (not a surprise), but...
print "You are worth $networth";	// prints: You are worth 0

[PHP only] PHP will recognize variable names inside "double quotes" and replace them with their values. Because they start with $. This is not true with 'single quotes', nor in C or Java.

All the comparisons:

==	is equal to
!=	not equal
<	less than
<=	less than or equal
>	greater than
>=	greater than or equal

if (...) ... else ...

You may ask, "What if I want to do some test, and then do one thing or another? Or more than 2 things. Well, any if statement may optionally have an else clause, to be done only when the condition is false.

if ($temperature > 0)
	print "It might rain";
else
	print "It might snow";

The second (false) statement can even be another if, giving an "else if", which can be formatted as follows:

if ($number == 12)
	print "That's a dozen";
else if ($number == 3 or $number == 7)
	print "$number is a lucky number";
else if ($number == 13)
	print "$number is an unlucky number";
else				// unconditional else is optional
	print "$number is boring";

What is TRUTH? [Bertrand Russell wrote a book about it.] In C, 0 (zero) is FALSE, all other numbers are TRUE. Comparisons give the value 0 or 1 (one, TRUE).
In PHP, every value is TRUE except: 0, 0.0, "0", "" (the empty string), undefined, FALSE, and NULL

Confused? So may be the poor stupid computer.

You can expect to get error messages if your syntax is not exact. For example, if you forget a semicolon, the computer gets lost and produces a cryptic error message on the next line. Mismatched brackets or quotes cause other muddles. Thankfully, modern text editors do syntax highlighting, and often show the matching bracket, pay attention to this.

There are lots of tutorial introductions on the web. For my part, go on to read about arrays, or directly to the howto.

Lin Jensen