1. if Statement

The if statement executes a block of code if a specified condition is true.

Syntax:

if (condition) {
    // Code to be executed if the condition is true
}

Example:

<?php
$age = 20;

if ($age >= 18) {
    echo "You are an adult.";
}
?>

In this example, if $age is 20, which is greater than or equal to 18, the message "You are an adult." is displayed.

2. if-else Statement

The if-else statement executes one block of code if the condition is true and another block if the condition is false.

Syntax:

if (condition) {
    // Code to be executed if the condition is true
} else {
    // Code to be executed if the condition is false
}

Example:

<?php
$age = 16;

if ($age >= 18) {
    echo "You are an adult.";
} else {
    echo "You are a minor.";
}
?>

Here, if $age is 16, the condition is false, so "You are a minor." is displayed.

3. if-elseif-else Statement

This statement allows you to check multiple conditions. If the first condition is false, it moves to the next condition, and so on.

Syntax:

if (condition1) {
    // Code to be executed if condition1 is true
} elseif (condition2) {
    // Code to be executed if condition2 is true
} else {
    // Code to be executed if both conditions are false
}

Example:

<?php
$score = 85;

if ($score >= 90) {
    echo "Grade: A";
} elseif ($score >= 80) {
    echo "Grade: B";
} elseif ($score >= 70) {
    echo "Grade: C";
} else {
    echo "Grade: F";
}
?>

The code checks multiple conditions to determine the grade based on $score.

4. switch Statement

The switch statement is used when you have multiple conditions based on the value of a single expression. It is often more readable than multiple if-elseif statements.

Syntax:

switch (expression) {
    case value1:
        // Code to be executed if expression == value1
        break;
    case value2:
        // Code to be executed if expression == value2
        break;
    // More cases...
    default:
        // Code to be executed if no case matches
}

Example:

<?php
$day = 3;

switch ($day) {
    case 1:
        echo "Monday";
        break;
    case 2:
        echo "Tuesday";
        break;
    case 3:
        echo "Wednesday";
        break;
    case 4:
        echo "Thursday";
        break;
    case 5:
        echo "Friday";
        break;
    case 6:
        echo "Saturday";
        break;
    case 7:
        echo "Sunday";
        break;
    default:
        echo "Invalid day";
}
?>

In this example, since $day is 3, the code will output "Wednesday".

5. ternary Operator

The ternary operator is a shorthand way to perform a simple if-else check in a single line of code.

Syntax:

(condition) ? true_value : false_value;

Example:

<?php
$age = 20;
echo ($age >= 18) ? "Adult" : "Minor";
?>

Here, if $age is 20, which is greater than or equal to 18, it outputs "Adult"; otherwise, it outputs "Minor".

6. null coalescing Operator

The null coalescing operator (??) is used to return the value of its left operand if it exists and is not null; otherwise, it returns its right operand.

Syntax:

$value = $variable ?? default_value;

Example:

<?php
$username = $_GET['user'] ?? 'guest';
echo $username;
?>

If $_GET['user'] is not set, it defaults to 'guest'.