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The ^= operator is the bitwise XOR (exclusive OR) assignment operator in PHP. It performs a bitwise XOR operation between the variable on the left and the evaluated expression on the right, subsequently assigning the computed binary result back to the variable on the left. 
$a ^= $b;

// This is strictly equivalent to the expanded form:
$a = $a ^ $b;

​
Mechanical Behavior

When the ^= operator is executed, PHP converts both operands into their binary representations and evaluates them bit-by-bit. The XOR logic dictates the resulting bit for each position based on the following truth table:
  • If both bits are identical (1 and 1, or 0 and 0), the resulting bit is 0.
  • If the bits are different (1 and 0, or 0 and 1), the resulting bit is 1.

​
Integer Evaluation

When operating on integers, the bitwise comparison occurs on the standard binary integer representation. 
$x = 12; // Binary representation: 1100
$y = 5;  // Binary representation: 0101

$x ^= $y; 

/* 
 * Bitwise evaluation:
 *   1100  ($x)
 * ^ 0101  ($y)
 * ---
 *   1001  (Result: 9)
 */

echo $x; // Outputs: 9

​
String Evaluation

If both operands are strings, PHP alters its behavior and performs the bitwise XOR operation against the ASCII values of the corresponding characters in the strings. The operation evaluates the strings byte-by-byte. If the strings are of unequal length, the shorter string is implicitly padded with null bytes (\0) to match the length of the longer string before the bitwise evaluation occurs. 
$char = "A";     // ASCII 65 (Binary: 01000001)
$mask = " ";     // ASCII 32 (Binary: 00100000)

$char ^= $mask;  

/*
 * Bitwise evaluation:
 *   01000001 ("A")
 * ^ 00100000 (" ")
 * -------
 *   01100001 (Result: 97, which is ASCII "a")
 */

echo $char; // Outputs: "a"

​
Type Juggling Considerations

If one operand is a string and the other is an integer, PHP’s behavior depends on the contents of the string. As of PHP 8.0:
  • Integer Strings: If the string is a valid integer string (e.g., "10"), PHP will implicitly cast the string to an integer before performing the bitwise XOR operation. While float-strings (e.g., "10.5") are also evaluated, using them with bitwise operators in PHP 8.1+ will trigger a Deprecated: Implicit conversion from float-string to int is deprecated warning before performing the cast.
  • Non-Numeric Strings: If a non-numeric string is used with an integer, PHP does not perform an implicit cast. Instead, it halts execution and throws a TypeError.
$val = 5;
$val ^= "10"; // "10" is an integer string, cast to int(10). Result is 15.

$val ^= "abc"; // "abc" is non-numeric. Throws TypeError: Unsupported operand types: int ^ string
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