PHP lexical structure
Computer languages, like human languages, have a lexical structure. A source code of a PHP script consists of tokens. Tokens are atomic code elements. In PHP language, we have comments, variables, literals, operators, delimiters and keywords.Comments
Comments are used by humans to clarify the source code. All comments in PHP follow the # character.<?php
# comments.php
# author Jan Bodnar
# ZetCode 2009
echo "This is comments.php script\n";
?>
Everything that follows the # character is ignored by the PHP interpreter.
// comments.php
// author Jan Bodnar
// ZetCode 2009
/*
comments.php
author Jan Bodnar
ZetCode 2009
*/
PHP also recognizes the comments from the C language.
White space
White space in PHP is used to separate tokens in PHP source file. It is used to improve readability of the source code.public $isRunning;
White spaces are required in some places. For example between the access specifier and the variable name. In other places, it is forbidden. It cannot be present in variable identifiers.
$a=1;
$b = 2;
$c = 3;
The amount of space put between tokens is irrelevant for the PHP interpreter.
$a = 1;
$b = 2; $c = 3;
$d
=
4;
We can put two statements into one line. Or one statement into three lines. However, source
code should be readable for humans. There are accepted standards of
how to lay out your source code.
Semicolon
A semicolon is used to mark the end of a statement in PHP. It is mandatory.$a = 34;
$b = $a * 34 - 34;
echo $a;
Here we have three different PHP statements. The first is an assignment. It puts
a value into the $a variable. The second one is an expression. The expression is
evaluated and the output is given to the $b variable. The third one is a command.
It prints the $a variable.
Variables
Avariable is an identifier, which holds a value.
In programming we say, that we assign a value to
a variable. Technically speaking, a variable is a reference to a computer
memory, where the value is stored.
In PHP language, a variable can hold a string, a number or various objects
like a function or a class. Variables can be assigned
different values over time.
Variables in PHP consist of the $ character and a label. A label can be created from alphanumeric characters and an underscore (_) character. A variable cannot begin with a number. The PHP interpreter can then distinguish between a number and a variable more easily.
$Value
$value2
$company_name
These were valid identifiers.
$12Val
$exx$
$first-name
These were examples of invalid identifiers.
The variables are case sensitive. This means, that
$Price, $price, and $PRICE
are three different identifiers.
<?php
$number = 10;
$Number = 11;
$NUMBER = 12;
echo $number, $Number, $NUMBER;
echo "\n";
?>
In our script, we assign three numeric values to three variables and print them.
101112
This is the output of the script.
A literal
Aliteral is any notation for representing a
value within the PHP source code. Technically, a literal will be assigned a value
at compile time, while a variable will be assigned at runtime.
$age = 29;
$nationality = "Hungarian";
Here we assign two literals to variables. Number 29 and string Hungarian are literals.
<?php
$name1 = "Jane ";
$age1 = 17;
$name2 = "Rose ";
$age2 = 16;
echo "Patrick 34\n";
echo "Luke 22\n";
echo $name1, $age1, "\n";
echo $name2, $age2, "\n";
?>
If we do not assign a literal to a variable, there is no way, how we can
work with it. It is dropped.
$ php literals.php
Patrick 34
Luke 22
Jane 17
Rose 16
This is the output of the literals.php script.
Operators
Anoperator is a symbol used to perform an action
on some value. (answers.com)
+ - * / % ++ --
= += -= *= /= .= %=
== != >< > < >= <=
&& || ! xor or
& ^ | ~ . << >>
These are PHP operators. We will talk about operators later in the tutorial.
Delimiters
Adelimiter is a sequence of one or more characters used
to specify the boundary between separate, independent regions in plain
text or other data stream. (wikipedia)
$a = "PHP";
$b = 'Java';
The single and double characters are used to mark the beginning and the end of a string.
function setDate($date) {
$this->date = $data;
}
if ( $a > $b) {
echo "\$a is bigger than \$b";
}
Parentheses are used to mark the function signature. The signature is the function parameters.
Curly brackets are used to mark the beginning and the end of the function body. They are also used
in flow control.
$a = array(1, 2, 3);
echo $a[1];
The square brackets are used to mark the array index.
/*
Author Jan Bodnar
December 2009
ZetCode
*/
/* */ delimiters are used to provide C style comments in PHP.
<?php
// PHP code
?>
The <?php and ?> delimiters are used to delimit PHP code in a file.
Keywords
A keyword is a reserved word in the PHP programming language. Keywords are used to perform a specific task in the computer program. For example, print a value, do repetitive tasks or perform logical operations. A programmer cannot use a keyword as an ordinary variable.The following is a list of PHP keywords.
abstract and array() as break
case catch class clone const
continue declare default do else
elseif enddeclare endfor endforeach endif
endswitch endwhile extends final for
foreach function global goto if
implements interface instanceof namespace new
or private protected public static
switch throw try use var
while xor
The following is a list of PHP compile time constants.
__CLASS__ __DIR__ __FILE__ __FUNCTION__
__METHOD__ __NAMESPACE__
Next we have other language constructs.
die() echo() empty() exit() eval()
include() include_once() isset() list() require()
require_once() return() print() unset()
In this part of the PHP tutorial, we covered the basic lexis for the PHP
language.
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