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", 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);
}
?>