php - What's the purpose of this bitwise AND operator? -

the code basic php code cms,it read mysql content render web page based on 3 different methods ,by id,name or special type(index page,sitemap page ...).but can't understand line " $r=dbrow("select * pages special&$v limit 1");" bitwise , supposed ?

<?php class page{     static $instances             = array();     static $instancesbyname     = array();     static $instancesbyspecial   = array();     function __construct($v,$byfield=0,$fromrow=0,$pvq=0){         # byfield: 0=id; 1=name; 3=special         if (!$byfield && is_numeric($v)){ // id             $r=$fromrow?$fromrow:($v?dbrow("select * pages id=$v limit 1"):array());         }         else if ($byfield == 1){ // name             $name=strtolower(str_replace('-','_',$v));             $fname='page_by_name_'.md5($name);             $r=dbrow("select * pages name '".addslashes($name)."' limit 1");         }         else if ($byfield == 3 && is_numeric($v)){ // special             $fname='page_by_special_'.$v;             $r=dbrow("select * pages special&$v limit 1");         }         else return false;         if(!count($r || !is_array($r)))return false;         if(!isset($r['id']))$r['id']=0;         if(!isset($r['type']))$r['type']=0;         if(!isset($r['special']))$r['special']=0;         if(!isset($r['name']))$r['name']='no name supplied';         foreach ($r $k=>$v) $this->{$k}=$v;         $this->urlname=$r['name'];         $this->dbvals=$r;         self::$instances[$this->id] =& $this;         self::$instancesbyname[preg_replace('/[^a-z0-9]/','-',strtolower($this->urlname))] =& $this;         self::$instancesbyspecial[$this->special] =& $this;         if(!$this->vars)$this->vars='{}';         $this->vars=json_decode($this->vars);     }     function getinstance($id=0,$fromrow=false,$pvq=false){         if (!is_numeric($id)) return false;         if (!@array_key_exists($id,self::$instances)) self::$instances[$id]=new page($id,0,$fromrow,$pvq);         return self::$instances[$id];     }     function getinstancebyname($name=''){         $name=strtolower($name);         $nameindex=preg_replace('#[^a-z0-9/]#','-',$name);         if(@array_key_exists($nameindex,self::$instancesbyname))return self::$instancesbyname[$nameindex];         self::$instancesbyname[$nameindex]=new page($name,1);         return self::$instancesbyname[$nameindex];     }     function getinstancebyspecial($sp=0){         if (!is_numeric($sp)) return false;         if (!@array_key_exists($sp,$instancesbyspecial)) $instancesbyspecial[$sp]=new page($sp,3);         return $instancesbyspecial[$sp];     } } 

it appears pages.special contains bit field (that is, each bit position in values flag application-specific property).

then, find records have properties, 1 perform bitwise and operation desired "mask"—bit positions in result set only if same position set in both value , mask. complete truth-table 2-bit value this:

 +---------+------+------------+ |  value  | mask | value&mask | +---------+------+------------+ |    0b00 | 0b00 |       0b00 | |    0b01 | 0b00 |       0b00 | |    0b10 | 0b00 |       0b00 | |    0b11 | 0b00 |       0b00 | |    0b00 | 0b01 |       0b00 | |    0b01 | 0b01 |       0b01 | |    0b10 | 0b01 |       0b00 | |    0b11 | 0b01 |       0b01 | |    0b00 | 0b10 |       0b00 | |    0b01 | 0b10 |       0b00 | |    0b10 | 0b10 |       0b10 | |    0b11 | 0b10 |       0b10 | |    0b00 | 0b11 |       0b00 | |    0b01 | 0b11 |       0b01 | |    0b10 | 0b11 |       0b10 | |    0b11 | 0b11 |       0b11 | +---------+------+------------+ 

for example, suppose lowest-order bit flag respective page "is readonly"; , next bit page "requires moderation"—a value of (decimal) 2, or 0b10, indicate page not readonly does require moderation. so, find pages either readonly or require moderation, 1 do:

select * pages (special & 0b11) != 0 

because mysql doesn't have true boolean types (but instead treats 0 false , non-zero true), filter expression can abbreviated:

select * pages special & 0b11 

we state mask in decimal, rather binary, form:

select * pages special & 3 

and, if desired, apply limit query (although in absence of order by clause, result indeterminate):

select * pages special&3 limit 1 

given have work (from code have shown), can following:

the query selects columns of indeterminate record pages table value of special has @ least 1 of same bits set in mask $v.

since semantics of special , $v (and bit positions within them) application-specific, impossible 1 more without deeper understanding of application.

note that, whilst compact, filtering using mask on bit field not sargable—so tends poor database design.