Variables and Data Types

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3. Variables and Data Types

Declaring Variables

In PHP, a variable is a container used to store data that can change during the execution of a script. All variables in PHP start with a dollar sign ($) followed by the variable name. A variable name can include letters, numbers, and underscores, but it must start with a letter or an underscore.

Here’s how you declare a variable:

<?php
  $name = "John"; // String variable
  $age = 25;      // Integer variable
  $isStudent = true; // Boolean variable
?>

In the example above:

• $name is a variable that stores a string value.
• $age is an integer variable.
• $isStudent is a boolean variable (either true or false).

PHP Data Types

PHP supports several types of data. The most common ones are:

1. String: A sequence of characters enclosed in quotes (either single ' or double ").

   • Example: $name = "John Doe";

2. Integer: A whole number (positive or negative).

   • Example: $age = 30;

3. Float (or Double): A number with a decimal point.

   • Example: $price = 19.99;

4. Boolean: A true or false value.

   • Example: $isValid = true;

5. Array: A collection of values. You can store multiple items in a single variable.

   • Example:

     $colors = array("Red", "Green", "Blue");
     

6. Object: A data structure that can contain both data and functions (methods). We will explore objects in later sections when we dive into object-oriented programming.

7. NULL: A special data type that represents a variable with no value.

   • Example: $emptyVar = NULL;

Variable Naming Conventions

While naming variables, there are a few rules and conventions to keep in mind:

• A variable name must start with a letter or underscore (_), followed by letters, numbers, or underscores.
• Variable names are case-sensitive. For example, $Name and $name are different variables.
• It's good practice to use descriptive names for your variables to make the code easier to understand.

Example of good variable names:

<?php
  $firstName = "John";
  $lastName = "Doe";
  $isStudent = true;
?>

Avoid using vague names like:

<?php
  $x = "John";
  $y = 25;
?>

Type Casting and Type Juggling

PHP automatically converts (or "casts") data from one type to another when necessary. This is called type juggling. For example, if you try to add a string and an integer, PHP will convert the string to a number if possible.

Example of type juggling:

<?php
  $num = "5"; // string
  $sum = $num + 10; // $num is automatically converted to an integer
  echo $sum; // Outputs: 15
?>

However, if you need to explicitly convert a variable from one type to another, you can use type casting:

<?php
  $str = "123.45";
  $int = (int) $str; // Cast string to integer
  echo $int; // Outputs: 123
?>

Constants

In PHP, constants are variables that cannot be changed once they are defined. You define a constant using the define() function.

Example:

<?php
  define("PI", 3.14159);
  echo PI; // Outputs: 3.14159
?>

Unlike variables, constants do not start with a dollar sign ($) and are case-sensitive by default.

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Summary of Variables and Data Types:

• PHP variables start with $ and can hold different types of data like strings, integers, floats, booleans, arrays, and objects.
• PHP supports automatic type conversion (type juggling), but you can also explicitly cast data types.
• Constants are similar to variables but cannot be changed once defined.

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