Pre-Increment

The ++ operator is a unary mutating operator in PHP that increments the value of a variable by one. It directly modifies the operand in memory and evaluates to either the original or the incremented value, depending on its syntactic placement relative to the variable.

Evaluation Modes

The operator functions in two distinct modes based on its position:

1. Pre-increment (++$variable) The variable is incremented first, and the expression evaluates to the new mutated value.

$x = 10;
$result = ++$x; 

// $x is mutated to 11
// $result is assigned 11

2. Post-increment ($variable++) The expression evaluates to the original value of the variable, and the variable is incremented afterward.

$y = 10;
$result = $y++; 

// $result is assigned 10
// $y is mutated to 11

Type-Specific Behavior and Coercion

Unlike standard binary addition ($var + 1), the ++ operator does not strictly cast all types to integers or floats. It applies specific internal rules based on the operand’s underlying zval type:

• Integers and Floats: Performs standard arithmetic incrementation.
• Null: Coerces the value to 1.
• Booleans: The operator does not mutate the value (true remains true, and false remains false). As of PHP 8.3, applying the ++ operator to a boolean emits an E_WARNING.
• Strings: Behavior depends on the string’s contents:
  • Numeric strings: Strings containing valid numeric values (e.g., "5", "5.5") are coerced into int or float types and incremented arithmetically (e.g., "5" becomes int(6)).
  • Non-numeric strings: Historically, PHP applies a Perl-style alphanumeric increment where characters carry over (e.g., 'a' becomes 'b', 'Z' becomes 'AA'). As of PHP 8.3, applying the ++ operator to non-numeric strings is deprecated and emits an E_DEPRECATED notice.
• Arrays: Applying the ++ operator results in a TypeError.
• Objects: Applying the ++ operator to standard userland objects results in a TypeError. However, internal classes that support operator overloading at the engine level (such as GMP objects) can be successfully incremented.

Internal Execution

At the compilation level, the Zend Engine translates the ++ operator into specific opcodes (ZEND_PRE_INC or ZEND_POST_INC). Because these opcodes are designed to mutate a variable’s zval directly in place, the ++ operator requires a memory reference. It is strictly bound to variables and cannot be applied to literal values or expressions.

// Valid: Applied to a variable reference
$a = 5;
$a++;

// Valid: Applied to an internal object supporting operator overloading
$gmp = gmp_init(5);
$gmp++;

// Invalid: Parse error: syntax error, unexpected integer "5", expecting variable
++5;

// Invalid: Parse error: syntax error, unexpected token "++"
(2 + 3)++;

// Invalid: Fatal error: Cannot use temporary expression in write context
[1, 2, 3][0]++;

Master PHP with Deep Grasping Methodology!

Learn More