It's essentially a condensed and time-efficient way to swap the integer values of two variables. The $a ^= $b assignment operator is the short-hand version of $a = $a ^ $b, while ^ is the operator for the bitwise XOR function. The reason it works is deeply rooted in the XOR function's property of forming distinct hybrids of two parameters that allow the reconstruction of each parameter if the hybrid is known (which is why it was once popular in simple encryption). Read a good and longer explanation here: https://betterexplained.com/articles/swap-two-variables-using-xor/

However, above solution was only chosen for brevity and is not suitable for deployment as it's not quite readable and comes with caveats such as it's only properly working for integers. If you put in strings of different lengths, it will produce scrambled versions of them but not the proper ones.