bindec

Two functions to convert 16bit or 8bit binary to integer using two's complement. If input exceeds maximum bits, false is returned. Function is easily scalable to x bits by changing the hexadecimals.
<?php function _bin16dec($bin) {
 // Function to convert 16bit binary numbers to integers using two's complement
 $num = bindec($bin);
 if($num > 0xFFFF) { return false; }
 if($num >= 0x8000) {
 return -(($num ^ 0xFFFF)+1);
 } else {
 return $num;
 }
}
function _bin8dec($bin) {
 // Function to convert 8bit binary numbers to integers using two's complement
 $num = bindec($bin);
 if($num > 0xFF) { return false; }
 if($num >= 0x80) {
 return -(($num ^ 0xFF)+1);
 } else {
 return $num;
 }
} ?>

## calculate binary with "shift-method" ##
<?php
function dec2bin($decimal_code){
 for($half=($decimal_code);$half>=1;$half=(floor($half))/2){
 if(($half%2)!=0){
 $y.=1;
 }
 else{
 $y.=0;
 }
 }
 $calculated_bin=strrev($y);
 return $calculated_bin;
}
?>

## example ##
[bin] 123 = [dec] 1111011
e.g.
123/2 = 61,5 => 1
61/2 = 30,5 => 1
30/2 = 15 => 0
15/2 = 7,5 => 1
7/2 = 3,5 => 1
3/2 = 1,5 => 1
1/2 = 0,5 => 1
(0/2 = 0 finish)

i think a better method than the "shift-method" is my method ^^...
here it comes:
function convert2bin($string) {
 $finished=0;
 $base=1;
 if(preg_match("/[^0-9]/", $string)) {
 for($i=0; $string!=chr($i); $i++);
 $dec_nr=$i;
 }
 else $dec_nr=$string;
 while($dec_nr>$base) {
 $base=$base*2;
 if($base>$dec_nr) {
 $base=$base/2;
 break;
 }
 }
 while(!$finished) {
 if(($dec_nr-$base)>0) {
 $dec_nr=$dec_nr-$base;
 $bin_nr.=1;
 $base=$base/2;
 }
 elseif(($dec_nr-$base)<0) {
 $bin_nr.=0;
 $base=$base/2;
 }
 elseif(($dec_nr-$base)==0) {
 $bin_nr.=1;
 $finished=1;
 while($base>1) { 
 $bin_nr.=0;
 $base=$base/2;
 }
 }
 }
 return $bin_nr;
 }
=====================================================
if you want to reconvert it (from binary to string or integer) you can use this function: 
function reconvert($bin_nr) {
 $base=1;
 $dec_nr=0;
 $bin_nr=explode(",", preg_replace("/(.*),/", "1ドル", str_replace("1", "1,", str_replace("0", "0,", $bin_nr))));
 for($i=1; $i<count($bin_nr); $i++) $base=$base*2;
 foreach($bin_nr as $key=>$bin_nr_bit) {
 if($bin_nr_bit==1) {
 $dec_nr+=$base;
 $base=$base/2;
 }
 if($bin_nr_bit==0) $base=$base/2;
 }
 return(array("string"=>chr($dec_nr), "int"=>$dec_nr));
 }

Binary to Decimal conversion using the BCMath extension..
<?php
function BCBin2Dec($Input='') {
 $Output='0';
 if(preg_match("/^[01]+$/",$Input)) {
 for($i=0;$i<strlen($Input);$i++)
 $Output=BCAdd(BCMul($Output,'2'),$Input{$i});
 }
 return($Output);
}
?>

This will simply convert from Base-2 to Base-10 using BCMath (arbitrary precision calculation).
See also: my 'BCDec2Bin' function on the 'decbin' document.
Enjoy,
Nitrogen.

The "smartbindec" function I wrote below will convert any binary string (of a reasonable size) to decimal. It will use two's complement if the leftmost bit is 1, regardless of bit length. If you are getting unexpected negative answers, try zero-padding your strings with sprintf("%032s", $yourBitString).
<?php
function twoscomp($bin) {
 $out = "";
 $mode = "init";
 for($x = strlen($bin)-1; $x >= 0; $x--) {
 if ($mode != "init")
 $out = ($bin[$x] == "0" ? "1" : "0").$out;
 else {
 if($bin[$x] == "1") {
 $out = "1".$out;
 $mode = "invert";
 }
 else
 $out = "0".$out;
 }
 }
 return $out;
}
function smartbindec($bin) {
 if($bin[0] == 1)
 return -1 * bindec(twoscomp($bin));
 else return (int) bindec($bin);
}
?>