In PHP, a variable is declared using a $ sign followed by the variable name. Here, some important points to know about variables:

• As PHP is a loosely typed language, so we do not need to declare the data types of the variables. It automatically analyzes the values and makes conversions to its correct datatype.
• After declaring a variable, it can be reused throughout the code.
• Assignment Operator (=) is used to assign the value to a variable.

Syntax of declaring a variable in PHP is given below:

$variablename=value;

Rules for declaring PHP variable:

• A variable must start with a dollar ($) sign, followed by the variable name.
• It can only contain alpha-numeric character and underscore (A-z, 0-9, _).
• A variable name must start with a letter or underscore (_) character.
• A PHP variable name cannot contain spaces.
• One thing to be kept in mind that the variable name cannot start with a number or special symbols.
• PHP variables are case-sensitive, so $name and $NAME both are treated as different variable.

PHP Variable: Declaring string, integer, and float

Let's see the example to store string, integer, and float values in PHP variables.

File: variable1.php

<?php
$str="hello string";
$x=200;
$y=44.6;
echo "string is: $str 
";
echo "integer is: $x 
";
echo "float is: $y 
";
?>

Output:

string is: hello string
integer is: 200
float is: 44.6 

PHP Variable: Sum of two variables

File: variable2.php

<?php
$x=5;
$y=6;
$z=$x+$y;
echo $z;
?>

Output:

11

PHP Variable: case sensitive

In PHP, variable names are case sensitive. So variable name "color" is different from Color, COLOR, COLor etc.

File: variable3.php

<?php
$color="red";
echo "My car is " . $color . "<br>";
echo "My house is " . $COLOR . "<br>";
echo "My boat is " . $coLOR . "<br>";
?>

Output:

My car is red
Notice: Undefined variable: COLOR in C:\wamp\www\variable.php on line 4
My house is 
Notice: Undefined variable: coLOR in C:\wamp\www\variable.php on line 5
My boat is 

PHP Variable: Rules

PHP variables must start with letter or underscore only.

PHP variable can't be start with numbers and special symbols.

File: variablevalid.php

<?php
$a="hello";//letter (valid)
$_b="hello";//underscore (valid)

echo "$a <br/> $_b";
?>

Output:

hello 
hello

File: variableinvalid.php

<?php
$4c="hello";//number (invalid)
$*d="hello";//special symbol (invalid)

echo "$4c <br/> $*d";
?>

Output:

Parse error: syntax error, unexpected '4' (T_LNUMBER), expecting variable (T_VARIABLE)
 or '$' in C:\wamp\www\variableinvalid.php on line 2

PHP: Loosely typed language

PHP is a loosely typed language, it means PHP automatically converts the variable to its correct data type.