Difference between the Ternary operator and Null coalescing operator in php

In PHP, the ternary operator (?:) and the null coalescing operator (??) are both shorthand for conditional expressions. The ternary operator evaluates a condition and returns one of two values, while the null coalescing operator specifically checks if a variable is set and not null.

Ternary Operator (?:)

The ternary operator replaces if-else statements into a single expression −

Syntax: (condition) ? expression1 : expression2;

Equivalent:
if (condition) {
    return expression1;
} else {
    return expression2;
}

If the condition is true, it returns expression1; otherwise it returns expression2.

Null Coalescing Operator (??)

The null coalescing operator (introduced in PHP 7) provides a default value when a variable is null or not set −

Syntax: $variable ?? expression;

Equivalent:
if (isset($variable)) {
    return $variable;
} else {
    return expression;
}

If the variable exists and is not null, it returns the variable's value; otherwise it returns the fallback expression. Unlike the ternary operator, it does not trigger an undefined variable notice.

Key Differences

Example

The following example demonstrates both operators ?

<?php
    // Null coalescing: no notice if variable doesn't exist
    $username = $_GET['username'] ?? 'not passed';
    echo "Null coalescing: " . $username . "
";

    // Equivalent using ternary with isset()
    $username = isset($_GET['username']) ? $_GET['username'] : 'not passed';
    echo "Ternary: " . $username . "
";

    // Key difference: falsy values
    $value = 0;
    echo "Ternary (0):         " . ($value ? $value : 'default') . "
";
    echo "Null coalescing (0): " . ($value ?? 'default') . "
";
?>

The output of the above code is ?

Null coalescing: not passed
Ternary: not passed
Ternary (0):         default
Null coalescing (0): 0

Notice that the ternary operator treats 0 as falsy and returns "default", while the null coalescing operator returns 0 because it is set and not null.

Conclusion

Use the ternary operator for general conditional expressions. Use the null coalescing operator when you specifically need to check if a variable exists and is not null, especially for handling optional parameters like $_GET, $_POST, or array keys without triggering undefined variable notices.