Elseif Clause

The elseif clause is a conditional control structure chained after an if block as part of a broader if statement. It executes its associated statement block only if the preceding if condition, and any preceding elseif conditions, evaluate to false, and its own condition evaluates to true.

Standard Syntax

In standard PHP syntax using curly braces, elseif is declared immediately following the closing brace of the preceding if or elseif block.

if ($expression1) {
    // Executes if $expression1 is true
} elseif ($expression2) {
    // Executes if $expression1 is false AND $expression2 is true
} elseif ($expression3) {
    // Executes if $expression1 and $expression2 are false AND $expression3 is true
} else {
    // Executes if all preceding expressions are false
}

Execution Mechanics

1. Sequential Evaluation: The PHP engine evaluates the expressions strictly in the order they appear.
2. Boolean Context: Every expression passed to an elseif is implicitly cast to a boolean value (true or false) prior to evaluation.
3. Short-circuiting: Once an if or elseif condition evaluates to true, its corresponding block is executed, and the engine immediately exits the entire control structure. Subsequent elseif conditions are never evaluated.

elseif vs. else if

PHP supports both the single-word elseif and the two-word else if. When using standard curly brace syntax, their behavior is functionally identical.

However, at the parser level, else if (two words) is treated as an entirely new if statement nested implicitly within the else block of the preceding condition.

// This:
if ($a) {
} else if ($b) {
}

// Is parsed identically to this:
if ($a) {
} else {
    if ($b) {
    }
}

Alternative Syntax Constraints

PHP offers an alternative syntax for control structures using colons (:) and an endif; terminator. When utilizing this alternative syntax, you must use the single-word elseif.

Using the two-word else if in the alternative syntax will result in a fatal parse error.

Valid Alternative Syntax:

if ($expression1):
    // statements
elseif ($expression2):
    // statements
else:
    // statements
endif;

Invalid Alternative Syntax (Throws Parse Error):

if ($expression1):
    // statements
else if ($expression2): // Parse error: syntax error, unexpected token "if", expecting ":"
    // statements
endif;

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