Default Clause

The default clause is a fallback execution branch within PHP’s switch statements and match expressions. It triggers exclusively when the evaluated subject expression fails to satisfy any of the explicitly defined case conditions or match arms.

Mechanics in switch Statements

In a switch control structure, the default clause acts as an optional catch-all block.

• Comparison: It is evaluated after all case conditions have been checked using loose comparison (==).
• Optionality: It is not strictly required. If omitted and no case matches, PHP simply exits the switch block and continues script execution.
• Positioning and Fall-through: Syntactically, default can be placed anywhere within the switch block. However, if placed before the final case, it requires a break statement to prevent fall-through execution into subsequent cases. Conventionally, it is placed at the end of the block.

switch ($expression) {
    case 'A':
        // Executes if $expression == 'A'
        break;
    case 'B':
        // Executes if $expression == 'B'
        break;
    default:
        // Executes if $expression matches neither 'A' nor 'B'
        break;
}

Mechanics in match Expressions (PHP 8.0+)

In a match expression, the default clause serves as the exhaustive fallback arm.

• Comparison: It is evaluated after all other arms have been checked using strict comparison (===).
• Exhaustiveness: match expressions are strictly exhaustive. If the subject expression does not match any defined arm and the default clause is omitted, PHP will throw an UnhandledMatchError at runtime.
• Positioning: Syntactically, the default arm can be placed anywhere within the match expression. Regardless of its position, it acts as the absolute fallback and is only triggered after all other arms have been evaluated. Defining multiple default arms within a single match expression will trigger a Fatal error.

$return_value = match ($expression) {
    'A' => 'Result A', // Evaluates if $expression === 'A'
    'B' => 'Result B', // Evaluates if $expression === 'B'
    default => 'Fallback Result', // Evaluates if no strict match is found
};

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