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.
PHP comments
Comments are used by humans to clarify the source code. All comments in PHP follow the # character.
<?php # comments.php # Author Shishir Kant Singh # SEED Group echo "This is comments.php script\n";
Everything that follows the # character is ignored by the PHP interpreter.
// comments.php // author Shishir Kant Singh // SEED Group /* comments.php author Shishir Kant Singh SEED Group */
PHP also recognizes the comments from the C language.
PHP white space
White space in PHP is used to separate tokens in PHP source file. It is used to improve the 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. It is based on the preferences and the style of a programmer.
$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.
PHP 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.
PHP variables
A variable 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, called a sigil, 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 PHP identifiers.
$12Val $exx$ $first-name
These were examples of invalid PHP identifiers.
The variables are case sensitive. This means that $Price, $price, and $PRICE are three different identifiers.
case.php <?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. However, for clarity reasons, it is not recommended to create variables which differ only in case; it is considered a poor practice.
$ php case.php 101112
This is the output of the script.
PHP constants
A constant is an identifier for a value which cannot change during the execution of the script. By convention, constant identifiers are always uppercase.
constants.php
<?php
define("SIZE", 300);
define("EDGE", 100);
#SIZE = 100;
echo SIZE;
echo EDGE;
echo "\n";
In the script, we define two constants.
define("SIZE", 300);
define("EDGE", 100);
Constants are created with the define() function.
# SIZE = 100;
Constants differ from variables; we cannot assign a different value to an existing constant. The script will fail if we uncomment the line.
echo SIZE; echo EDGE;
Constants do not use the dollar sigil character.
$ php constants.php 300100
This is the output of the constants script.
The following is a list of PHP compile time constants.
__CLASS__ __DIR__ __FILE__ __FUNCTION__ __METHOD__ __NAMESPACE__
PHP literal
A literal is any notation for representing a value within the PHP source code. Technically, a literal is assigned a value at compile time, while a variable is assigned at runtime.
$age = 29; $nationality = "Hungarian";
Here we assign two literals to variables. Number 29 and string “Hungarian” are literals.
literals.php <?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.
PHP operators
An operator is a symbol used to perform an action on some value.
+ - * / % ** ++ -- ?: ?? = += -= *= /= .= %= == != === !== <> > < >= <= <=> && || ! xor or & ^ | ~ . << >>
These are PHP operators. We will talk about operators later in the tutorial.
PHP delimiters
A delimiter is a sequence of one or more characters used to specify the boundary between separate, independent regions in plain text or other data stream.
$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 January 2019 ZetCode */
/* */ delimiters are used to provide C style comments in PHP.
<?php // PHP code
The <?php delimiter is used to declare the start of PHP code.
PHP keywords
A keyword is a reserved word in the PHP programming language. Keywords are used to perform a specific task in a 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 yield yield from
Next we have other language constructs.
die() echo() empty() exit() eval() include() include_once() isset() list() require() require_once() return() print() unset()
