Data Types

PHP has a total of eight data types which we use to construct our variables −

• Integers − are whole numbers, without a decimal point, like 4195.
• Doubles − are floating-point numbers, like 3.14159 or 49.1.
• Booleans − have only two possible values either true or false.
• NULL − is a special type that only has one value: NULL.
• Strings − are sequences of characters, like 'PHP supports string operations.'
• Arrays − are named and indexed collections of other values.
• Objects − are instances of programmer-defined classes, which can package up both other kinds of values and functions that are specific to the class.
• Resources − are special variables that hold references to resources external to PHP (such as database connections).
  
  

  Integers

  They are whole numbers, without a decimal point, like 4195. They are the simplest type .they correspond to simple whole numbers, both positive and negative. Integers can be assigned to variables, or they can be used in expressions, like so −
  

  $int_var = 12345;
  $another_int = -12345 + 12345;

  Integer can be in decimal (base 10), octal (base 8), and hexadecimal (base 16) format. Decimal format is the default, octal integers are specified with a leading 0, and hexadecimals have a leading 0x.
  For most common platforms, the largest integer is (2**31 . 1) (or 2,147,483,647), and the smallest (most negative) integer is . (2**31 . 1) (or .2,147,483,647).
  

  Doubles

  They like 3.14159 or 49.1. By default, doubles print with the minimum number of decimal places needed. For example, the code −
  

  <?php
     $many = 2.2888800;
     $many_2 = 2.2111200;
     $few = $many + $many_2;
     
     print("$many + $many_2 = $few <br>");
  ?>

  It produces the following browser output −
  

  2.28888 + 2.21112 = 4.5
  

  Boolean

  They have only two possible values either true or false. PHP provides a couple of constants especially for use as Booleans: TRUE and FALSE, which can be used like so −
  

  if (TRUE)
     print("This will always print<br>");
  
  else
     print("This will never print<br>");

  Interpreting other types as Booleans

  Here are the rules for determine the "truth" of any value not already of the Boolean type −
  
• If the value is a number, it is false if exactly equal to zero and true otherwise.
• If the value is a string, it is false if the string is empty (has zero characters) or is the string "0", and is true otherwise.
• Values of type NULL are always false.
• If the value is an array, it is false if it contains no other values, and it is true otherwise. For an object, containing a value means having a member variable that has been assigned a value.
• Valid resources are true (although some functions that return resources when they are successful will return FALSE when unsuccessful).
• Don't use double as Booleans.

Each of the following variables has the truth value embedded in its name when it is used in a Boolean context.

$true_num = 3 + 0.14159;
$true_str = "Tried and true"
$true_array[49] = "An array element";
$false_array = array();
$false_null = NULL;
$false_num = 999 - 999;
$false_str = "";

NULL

NULL is a special type that only has one value: NULL. To give a variable the NULL value, simply assign it like this −

$my_var = NULL;

The special constant NULL is capitalized by convention, but actually it is case insensitive; you could just as well have typed −

$my_var = null;

A variable that has been assigned NULL has the following properties −

• It evaluates to FALSE in a Boolean context.
• It returns FALSE when tested with IsSet() function.

Strings

They are sequences of characters, like "PHP supports string operations". Following are valid examples of string

$string_1 = "This is a string in double quotes";
$string_2 = 'This is a somewhat longer, singly quoted string';
$string_39 = "This string has thirty-nine characters";
$string_0 = ""; // a string with zero characters

Singly quoted strings are treated almost literally, whereas doubly quoted strings replace variables with their values as well as specially interpreting certain character sequences.

<?php
   $variable = "name";
   $literally = 'My $variable will not print!';
   
   print($literally);
   print "<br>";
   
   $literally = "My $variable will print!";
   print($literally);
?>

This will produce following result −

My $variable will not print!\n
My name will print

There are no artificial limits on string length - within the bounds of available memory, you ought to be able to make arbitrarily long strings.
Strings that are delimited by double quotes (as in "this") are preprocessed in both the following two ways by PHP −

• Certain character sequences beginning with backslash (\) are replaced with special characters
• Variable names (starting with $) are replaced with string representations of their values.

The escape-sequence replacements are −

• \n is replaced by the newline character
• \r is replaced by the carriage-return character
• \t is replaced by the tab character
• \$ is replaced by the dollar sign itself ($)
• \" is replaced by a single double-quote (")
• \\ is replaced by a single backslash (\)

Variables

A variable is a special container that you can define to "hold" a value. A variable
consists of a name that you can choose, preceded by a dollar ($) sign. The variable
name can include letters, numbers, and the underscore character (_). Variable
names cannot include spaces or characters that are not alphanumeric. The following
code defines some legal variables:
$a;
$a_longish_variable_name;
$2453;
$sleepyZZZZ
Remember that a semicolon (;) is used to end a PHP statement. The semicolons in
the previous fragment of code are not part of the variable names.
NEW
TERM
A variable is a holder for a type of data. It can hold numbers, strings of
characters, objects, arrays, or booleans. The contents of a variable can
be changed at any time.
As you can see, you have plenty of choices about naming, although it is unusual to
see a variable name that consists exclusively of numbers. To declare a variable, you
need only to include it in your script. You usually declare a variable and assign a
value to it in the same statement.
$num1 = 8;
$num2 = 23;
The preceding lines declare two variables, using the assignment operator (=) to
give them values. You will learn about assignment in more detail in the Operators
and Expressions section later in the hour. After you give your variables values, you
can treat them exactly as if they were the values themselves. In other words
print $num1;
is equivalent to
print 8;
as long as $num1 contains 8.
Dynamic Variables
As you know, you create a variable with a dollar sign followed by a variable name.
Unusually, the variable name can itself be stored in a variable. So, when assigning
a value to a variable
$user = "bob";
is equivalent to
$holder="user";
$$holder = "bob";
The $holder variable contains the string "user", so you can think of $$holder as a
dollar sign followed by the value of $holder. PHP interprets this as $user.
Note You can use a string constant to define a dynamic variable instead of a
variable. To do so, you must wrap the string you want to use for the variable
name in braces:
${"user"} = "bob";
This might not seem useful at first glance. However, by using the
concatenation operator and a loop (see Hour 5, "Going with the Flow"), you
can use this technique to create tens of variables dynamically.
When accessing a dynamic variable, the syntax is exactly the same:
$user ="bob";
print $user;
is equivalent to
$user ="bob";
$holder="user";
print $$holder;
If you want to print a dynamic variable within a string, however, you need to give
the interpreter some help. The following print statement:
$user="bob";
$holder="user";
print "$$holder";
does not print "bob" to the browser as you might expect. Instead it prints the strings
"$" and "user" together to make "$user". When you place a variable within
quotation marks, PHP helpfully inserts its value. In this case, PHP replaces $holder
with the string "user". The first dollar sign is left in place. To make it clear to PHP
that a variable within a string is part of a dynamic variable, you must wrap it in
braces. The print statement in the following fragment:
$user="bob";
$holder="user";
print "${$holder}";
now prints "bob", which is the value contained in $user.
Listing 4.1 brings some of the previous code fragments together into a single script
using a string stored in a variable to initialize and access a variable called $user.
Example:
<html>
 <head>
 <title> Dynamically setting and accessing variables</title>
 </head>
 <body>
 <?php
 $holder = "user";
 $$holder = "bob";
 // could have been:
 // $user = "bob";
 // ${"user"} = "bob";
 print "$user<br>"; // prints "bob"
 print $$holder; // prints "bob"
 print "<br>";
 print "${$holder}<br>"; // prints "bob"
print "${'user'}<br>"; // prints "bob"
 ?>
 </body>
 </html>

output:-
bob
bob
bob
bob

How to compile and run a php program?

1) Install the XAMPP Server in your computer C:\
2) Type the following php program in a notepad

<html>
<body>
<?php
echo"Hello world, welcome to PHP!"
?>
</body>
</html>

3) save the file in C:\xampp\htdocs\yourfolder with name welcome.php

4) Now start XAMPP Server control panel and start apache server, MySql

5) Open the browser, in the addressbar type localhost/yourfolder name and press enter
it will show index of programs list
6) click on welcome.php link

7) Now it will show the output as "Hello world, welcome to PHP!"