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The ++ (increment) operator is a unary arithmetic operator that increases the value of a variable by one. It mutates the operand in place and evaluates to either the original or the incremented value, depending on its syntactic placement relative to the variable. The operator requires an assignable variable (an l-value) as its operand. Applying it to literal values or expressions (e.g., ++5 or ++($a + $b)) results in a fatal parse error.

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Evaluation Modes

The operator operates in two distinct modes based on its position: 1. Pre-increment (++$variable) The variable is mutated first, and the expression evaluates to the newly incremented value. 
$x = 10;
$y = ++$x; 

// State: $x is 11, $y is 11
2. Post-increment ($variable++) The expression evaluates to the variable’s current value, and the mutation occurs immediately afterward. 
$a = 10;
$b = $a++; 

// State: $a is 11, $b is 10

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Type-Specific Behavior and Coercion

Because PHP is dynamically typed, the ++ operator applies specific coercion rules depending on the operand’s internal type:
  • Integers and Floats: Performs standard numeric addition ($var + 1).
  • Null: Incrementing null coerces the value to an integer, resulting in 1.
  • Booleans: The operator does not modify boolean values (true remains true, and false remains false). Note: As of PHP 8.3, applying the ++ operator to a boolean value emits an E_WARNING.
  • Strings:
    • Numeric strings (e.g., "13") are cast to integers or floats and incremented numerically.
    • Alphanumeric strings historically follow Perl-style character incrementation (e.g., "a" becomes "b", "Z" becomes "AA").
    • Note: As of PHP 8.3, applying the ++ operator to non-numeric strings emits an E_DEPRECATED warning, and this behavior is slated for removal in PHP 9.0.
// Null behavior
$val = null;
$val++; // $val is now 1

// Boolean behavior (PHP 8.3+ emits E_WARNING)
$bool = true;
$bool++; // $bool remains true

// String behavior (PHP 8.3+ emits E_DEPRECATED for non-numeric)
$str = "W";
$str++; // $str is now "X"

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Internal Execution Order

When used within complex expressions, the post-increment operator’s delayed mutation is resolved before the execution moves to the next statement, but after the current sub-expression is evaluated. 
$i = 1;
$result = $i++ + $i++; 

// Evaluation sequence:
// 1. First $i++ evaluates to 1. $i mutates to 2.
// 2. Second $i++ evaluates to 2. $i mutates to 3.
// 3. $result = 1 + 2 = 3.
(Note: Since the introduction of the Abstract Syntax Tree (AST) in PHP 7.0 and strict evaluation order definitions in PHP 8.0, left-to-right evaluation is strictly guaranteed by the language. However, mutating the same variable multiple times in a single expression remains a poor coding practice that reduces readability).
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