Checking Bits With PHP
PHP makes life a lot easier for quick or dirty maintenance scripts, cron jobs or web applications but how does it do for older, not so straight forward problems dealing with bits and bytes? I was surprised how easy it was to manipulate bits in a byte with php. Here is an function that made my life a fair amount easier when having to check for a specific bit in a byte.
This function checks whether a certain bit is set or not given a byte and an index. It returns true if the chosen bit is set. It casts the $value argument to a integer just in case. The index $n goes from left to right so the most significant bit is bit one and the least significant is bit eight. This function will only work for integers between 0 and 255 because that was all I needed at the time. It would be trivial to write either a function to separate bytes in an integer or to increase the amount of bits that this function checks. I originally had a different function here but the Internet quickly told me that there was an easier way to do this.
<?php
function check_bit($value,$n=8)
{
$value = (int)$value;
if($value & (1<<(8-$n))) { return true; }
else { return false; }
}
//Check Bit Usage Example
$test_byte1 = "4";
if( check_bit($test_byte1,2))
echo "Bit 2 is set in ".decbin($test_byte1);
else
echo "Bit 2 is not set in ".decbin($test_byte1);
?>
Here are the php bitwise operator definitions from the php documentation. Look like C much?
| $a & $b | And | Bits that are set in both $a and $b are set. |
| $a | $b | Or | Bits that are set in either $a or $b are set. |
| $a ^ $b | Xor | Bits that are set in $a or $b but not both are set. |
| ~ $a | Not | Bits that are set in $a are not set, and vice versa. |
| $a << $b | Shift left | Shift the bits of $a $b steps to the left (each step means “multiply by two”) |
| $a >> $b | Shift right | Shift the bits of $a $b steps to the right (each step means “divide by two”) |
