What does it mean in PHP?

The << operator is not only in php, but in many other languages, it is a bit-to-bit operator .

What it does is to shift the value to the left of the operator n times, where n is the value to the right of the operator. (Move left obviously)

For example:

$a = 2 << 3;
// Em binário temos 2 = 0000 0010, deslocando ele 3x
// obtemos 0001 0000, que resulta em 16
echo $a; // 16

If you have more questions, you can consult the php manual: link

When the right-hand operator is 31, the shift-left operator is used for a very specific purpose: find out whether the left operator is even or odd.

If even, the result of par << 31 will be 0. If it is odd, the result of impar << 31 will be -2147483648, the minimum value of integer types.

0
= 0 000 0000 0000 0000 0000 0000 0000 0000 em binario
= 0 000 0000 0000 0000 0000 0000 0000 0000 depois de shift left
= 0

1
= 0 000 0000 0000 0000 0000 0000 0000 0001 em binario
= 1 000 0000 0000 0000 0000 0000 0000 0000 depois de shift left
= -2147483648

2 =
= 0 000 0000 0000 0000 0000 0000 0000 0010 em binario
= 0 000 0000 0000 0000 0000 0000 0000 0000 depois de shift left
= 0

...

51 =
= 0 000 0000 0000 0000 0000 0000 0011 0011 em binario
= 1 000 0000 0000 0000 0000 0000 0000 0000 depois de shift left
= -2147483648

The logic is to shift to the left until all the digits disappear except the least significant. This digit indicates whether the original number is even or odd.

Note: Because they are "signed integers," the leftmost binary digit is used for the positive or negative sign.

Note: This answer assumes that integers are represented with 32 bits. According to my research, PHP represents 64-bit integers if the processor supports 64-bit instructions - in this case a% shift must be done%