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The >>= operator is the bitwise right shift assignment operator in PHP. It shifts the binary representation of the left-hand variable to the right by the number of bit positions specified by the right-hand expression, and immediately assigns the resulting integer back to the left-hand variable.

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Syntax

$a >>= $b;
This is functionally equivalent to the expanded assignment form: 
$a = $a >> $b;

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Technical Mechanics

  1. Type Coercion and Precision Loss: Before the shift operation occurs, PHP evaluates both operands as integers. Floating-point numbers are truncated. As of PHP 8.1, passing a float with a fractional part to a bitwise operator emits a Deprecated notice due to the implicit loss of precision.
  2. Bit Discarding: Bits that are shifted off the right boundary (the least significant bits) are permanently discarded.
  3. Arithmetic Right Shift (Sign Extension): PHP performs an arithmetic right shift, not a logical right shift. The empty bit positions created on the left (the most significant bits) are filled with copies of the original sign bit. This ensures that the sign of the integer (positive or negative) is preserved during the shift.
  4. Mathematical Equivalence: Shifting an integer right by $b positions is mathematically equivalent to performing integer division by 2b2^b (floor($a / (2 ** $b))).
  5. Error Conditions: If the right-hand operand (the shift amount) is a negative integer, the PHP engine throws an ArithmeticError.

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Execution Example

// Positive Integer Shift
$val = 12;         // Binary: 00000000 00000000 00000000 00001100
$val >>= 2;        // Shifts right by 2 positions
echo $val;         // Outputs: 3
                   // Binary: 00000000 00000000 00000000 00000011

// Negative Integer Shift (Two's Complement Sign Extension)
$neg = -12;        // Binary: 11111111 11111111 11111111 11110100
$neg >>= 2;        // Shifts right by 2 positions, filling left with 1s
echo $neg;         // Outputs: -3
                   // Binary: 11111111 11111111 11111111 11111101
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