A utility class to create, manipulate, and check bitmasks (use powers of 2 for your values!). Written and tested with php5.4; should be compatible with 5.1+. Exception-based error handling. Released under the MIT license. See example usage below class definition.

bitmask.php

<?php
/**
 * models a bitfield and provides manipulation & comparison methods.
 * 
 * @author Adrian Testa-Avila <AT@custom-anything.com>
 * @copyright 2012
 * @license MIT License [http://opensource.org/licenses/MIT]
 */
class Bitfield{

    /** int                     set() flags to add or remove bit from bitmask. */
    const setbit_off    = 0;
    const setbit_on     = 1;
    
    /** int                     current bitfield value. */
    private $_bitfield;
    
    /** int                     original (constructed) bitfield value. */
    private $_bitfield_original;
    
    /**
     * sets bitfield to an initial value.
     *
     * @throws InvalidArgumentException if $b is not an integer
     */
    public function __construct( $b ){
        if( !is_integer( $b ) ){
            throw new InvalidArgumentException( 'bitfield must be an integer',E_USER_ERROR );
            $b = 0;
        }
        $this->_bitfield = $this->_bitfield_original = $b;
    }
    
    /**
     * checks if the current bitfield value matches the original.
     *
     * @return bool                     true if current bitfield matches original; false otherwise.
     */
    public function changed(){
        return ($this->_bitfield !== $this->_bitfield_original);
    }
    
    /**
     * checks the current bitfield against a given value.
     *
     * @param int $b                    value to check against
     * @throws InvalidArgumentException if $b is not an integer
     * @return bool                     true if $b is contaned in current bitmask; false otherwise
     */
    public function contains( $b ){
        if( !is_integer( $b ) ){
            throw new InvalidArgumentException( 'bitmask values must be integers',E_USER_NOTICE );
            return false;
        }
        return (($this->_bitfield & $b) == $b);
    }
        
    /**
     * checks the entire current bitfield, optionally against a given value.
     *
     * @param int $b                    the bitfield to check against
     * @throws InvalidArgumentException if $b is not an integer
     * @return int|bool                 true if provided $b matches the current bitfield, false if not;
     *                                  the bitfield otherwise
     */
    public function equals( $b=null ){
        if( $b === null ){
            return $this->_bitfield;
        }
        if( !is_integer( $b ) ){
            throw new InvalidArgumentException( 'bitfield must be an integer',E_USER_NOTICE );
            return false;
        }
        return ($this->_bitfield === $b);
    }
    
    /**
     * turns bits on or off.
     *
     * @param int $b                    value to set/remove
     * @throws InvalidArgumentException if $b is not an integer
     * @throws InvalidArgumentException if $mode is not 1 or 0
     * @return bool                     true if value was modified; false otherwise
     */
    public function modify( $b,$mode=self::setbit_on ){
        if( !is_integer( $b ) ){
            throw new InvalidArgumentException( 'bitmask values must be integers',E_USER_NOTICE );
            return false;
        }
        if( ($mode != 1) && ($mode != 0) ){
            throw new InvalidArgumentException( "'mode' must be '1' or '0'",E_USER_NOTICE );
            return false;
        }
        $unmod = $this->_bitfield;
        $mode?
            $this->_bitfield = $this->_bitfield & ~$b | $b:
            $this->_bitfield = $this->_bitfield & ~$b
        ;
        return !($this->equals( $unmod ));
    }
    
    /**
     * replaces the current bitfield with a new one.
     *
     * @param int $b                    value to set
     * @throws InvalidArgumentException if $b is not an integer
     * @return bool                     true if new bitfield set; false otherwise
     */
    public function replace( $b ){
        if( !is_integer( $b ) ){
            throw new InvalidArgumentException( 'bitfield must be an integer',E_USER_NOTICE );
            return false;
        }
        $this->bitfield = $b;
        return true;
    }
    
    /** restores the original (constructed) bitfield value. */
    public function reset(){
        $this->_bitfield = $this->_bitfield_original;
    }
    
}

// EXAMPLE USAGE //
/*
print "creating a new bitmask from 1|4...\n";
$b = new bitfield( 1|4 );
print "-> b is set to ".$b->equals()."\n\n";

print "checking if b contains 4...\n";
print $b->contains( 4 )? "-> b contains 4\n": "-> b is missing 4\n";
print "adding 4...\ndid b change?  ";
print $b->modify( 4,true )? "-> yes\n\n": "-> no\n\n";

print "removing 4...\n";
$b->modify( 4,false );
print $b->contains( 4 )? "-> b now contains 4\n": "-> b is now missing 4\n\n";

print "checking if b contains 16...\n";
print $b->contains( 16 )? "-> b contains 16\n": "-> b is missing 16\n\n";

print "adding 16...\n";
$b->modify( 16,true );
print $b->contains( 16 )? "-> b now contains 16\n": "-> b is now missing 16\n\n";

print "checking if the current b matches the original b...\n";
print $b->changed()? "-> b has changed from initial value\n\n": "-> b is still the same\n\n";

print "replacing b with 2|8|32...\n";
$b->replace( 2|8|32 );
print "-> b is now ".$b->equals()."\n\n";

print "resetting b to original value...\n";
$b->reset();
print "-> b is now reset to ".$b->equals();

/* outputs:

creating a new bitmask from 1|4...
-> b is set to 5

checking if b contains 4...
-> b contains 4
adding 4...
did b change?  -> no

removing 4...
-> b is now missing 4

checking if b contains 16...
-> b is missing 16

adding 16...
-> b now contains 16
checking if the current b matches the original b...
-> b has changed from initial value

replacing b with 2|8|32...
-> b is now 17

resetting b to original value...
-> b is now reset to 5
*/