Left Shift Assignment

The <<= operator is the bitwise left shift assignment operator in PHP. It shifts the binary representation of the left operand to the left by the number of bit positions specified by the right operand, and then assigns the resulting integer back to the left operand.

Syntax

$a <<= $b;

This operation is the shorthand equivalent of:

$a = $a << $b;

Technical Mechanics

• Type Casting: Before the shift operation occurs, PHP implicitly casts both operands to integers. Floating-point numbers are truncated, and strings are converted to integers based on standard PHP type juggling rules.
• Zero-Padding: As the bits are shifted to the left, the vacated bit positions on the right are filled with zeros (0).
• Signed Integers and the Sign Bit: PHP only supports signed integers and lacks an unsigned integer type. The most significant bit (the 64th bit on a 64-bit system) acts as the sign bit. If a 1 is shifted into this position, the integer’s sign changes, resulting in a negative number. For example, 1 << 63 evaluates to -9223372036854775808. This is a critical mechanic when building bitmasks in PHP.
• Overflow and Wrapping: Unlike standard arithmetic operations in PHP, which automatically promote overflowing integers to floating-point numbers, bitwise operations strictly yield integers. Exceeding PHP_INT_MAX via a left-shift pushes a 1 into the sign bit, wrapping the value to a negative integer. Any bits shifted past the system’s maximum boundary (past the sign bit) are permanently discarded on subsequent shifts.
• Mathematical Equivalence: Shifting an integer left by $b positions is mathematically equivalent to multiplying the integer by 2 to the power of $b ($a * (2 ** $b)), provided that the operation does not push bits into the sign bit or exceed the system’s integer bounds.

Edge Cases and Exceptions

• Negative Shifts: Shifting by a negative number of positions (e.g., $a <<= -1) is invalid in PHP and throws an ArithmeticError.
• Oversized Shifts: Shifting by an amount greater than or equal to the system’s integer bit width (e.g., shifting by 64 on a 64-bit system) does not safely shift all bits to 0. Instead, it results in architecture-dependent behavior. On most architectures, the shift amount is wrapped modulo the bit width. For example, a shift by 64 on a 64-bit system is evaluated as 64 % 64, resulting in an actual shift of 0 positions.

Execution Examples

Standard Left Shift:

$val = 5; 
// 64-bit Binary representation: 
// 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000101

$val <<= 3; 
// Shifts the bits 3 positions to the left.
// Vacated rightmost bits are padded with 000.
// New Binary representation:
// 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00101000

echo $val; 
// Output: 40 (which is 5 * (2 ** 3))

Sign Bit Wrapping (64-bit system):

$mask = 1;
// 64-bit Binary representation:
// 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001

$mask <<= 63;
// Shifts the 1 into the most significant bit (the sign bit).
// New Binary representation:
// 10000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000

echo $mask;
// Output: -9223372036854775808

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