PHP Programming Basics
PHP programs are files containing HTML tags with PHP commands
embedded in them. This is in contrast to many other dynamic web-page
solutions, which are scripts that generate HTML. The web server
processes the PHP commands and sends their output (and any HTML from
the file) to the browser.
The following program is a simple PHP program that displays the
message “Hello, World!”:
<html>
<head>
<title>Look out World</title>
</head>
<body>
<?php echo ‘Hello, World!’ ?>
</body>
</html>
In this example, the PHP echo command produces output (the string
“Hello, World!”) which is inserted in the output file. The tags
<?php and ?> encloses the PHP commands, which are
processed by the web server to generate the required HTML page.
Language Basics:
PHP is an Open Source Scripting language, which is strongly
influenced by other languages, such as Perl and C. A statement in
PHP is a collection of PHP code that does something. It can be as
simple as a variable assignment or as complicated as a loop with
multiple exit points.
PHP uses semicolons to separate simple statements. A compound
statement that uses curly braces to mark a block of code, such as
conditional test or loop, doesn’t need a semicolon after the
closing brace.
PHP is a case-insensitive language. It doesn’t consider the case
for built-in constructs and keywords. Thus, the following three
lines of code are equivalent in PHP:
echo(“Hello, World!”);
ECHO(“Hello, World!”);
EcHo(“Hello, World!”);
The names of user-defined classes and functions are also
case-insensitive in PHP. But for the ordinary variables (identifiers
used for naming memory locations), PHP becomes case-sensitive. That
is,
$name $NAME $NaMe
are three different variables in PHP.
Literals and Identifiers:
A literal is a data value that appears directly in a program. The
following are literals in PHP:
2001
0xFE
1.4142
‘Hi’
“Hello, World!”
true
null
An identifier is simply a name. In PHP, identifiers are used to name
variables, functions, constants and classes. Variable names always
begin with a dollar sign ($) and are case-sensitive. Where as,
function names and class names are case-insensitive.
The rules for forming a valid identifier in PHP are as follows:
- The first character of an identifier must be an ASCII letter (uppercase or lowercase), the underscore (_), or any of the characters between ASCII 0x7F and ASCII 0xFF.
- After the initial character, we can have characters from alphabets, numbers or special characters (except white space).
Some valid variable names are given below:
$bill
$head_count
$MaxForce
$_underscore
$_int
Here are some illegal variables names:
$not valid
$3Wa
Constants:
A constant is an identifier given for a simple value; only scalar
values – Boolean, integer, double and string – can be constants.
A constant can be defined using the function define() as
follows:
define(‘PUBLISHER’, “O’ Reilly & Associates”);
echo PUBLISHER;
The above set of statements defines constant named PUBLISHER, which
is used in the second statement for displaying its content “O’
Reilly & Associates”.
Once set, the value of a constant can’t be changed with in the
same program. But they can be referenced later by their identifier
any number of times in a program. The main advantage of using
constants is that their value can be changed, if required by changing
the value only in its definition.
Keywords:
A keyword is a word reserved by the language for its core
functionality. We can’t make use of keywords as identifiers for
variables, functions or classes. Some of the keywords defined in PHP
are:
and
array()
break
class
case
catch
endif
exit()
echo()
if
All keywords in PHP are case-insensitive.
