Description:
------------
See https://github.com/php/pecl-numbers-bitset/blob/master/bitset.c#L279 : the 
loop that 
performs the AND looks like this:

for (; i < to_bits; i++) {
	intern->bitset_val[i / CHAR_BIT] &= param->bitset_val[i / CHAR_BIT];
}

so taking, for example, CHAR_BIT=8, it does
 (intern[0] & param[0]),
 1/8=0 -> (intern[0] & param[0])
 2/8=0 -> (intern[0] & param[0])
 ...
 8/8=1 -> (intern[1] & param[1])

ie, it does AND CHAR_BIT times for every char. My suggested fix is to change the 
loop to 
something like this:

for (; i < to_bits; i+= CHAR_BIT) {
	intern->bitset_val[i / CHAR_BIT] &= param->bitset_val[i / CHAR_BIT];
}

//last increment may have set i above the bitset(s) bounds(and made the for loop 
skip 
last bits): operate on the last octect bit by bit

i = (to_bits/CHAR_BIT) * CHAR_BIT; // get the index of the last bit on a char 
boundary

for (unsigned char mask=0x0; i < to_bits; i++) {
        // bitwise and one bit at a time
        // please note the following code is not tested

        mask = ~(1 << (i%CHAR_BITS)); // all bits set to 1 except the current 
index
        mask |= param->bitset_val[i / CHAR_BIT]; // all 1s if the current index 
is set, 
mask unchanged if not

	intern->bitset_val[i / CHAR_BIT] &= mask; //changes only the current bit
}