php防sql注入原理

$sql = "select count(*) as ctr from users where username
='".mysql_real_escape_string($username)."' and
password='". mysql_real_escape_string($pw)."' limit 1";

function check_param($value=null) { 
 #select|insert|update|delete|\'|\/\*|\*|\.\.\/|\.\/|union|into|load_file|outfile
$str = 'select|insert|and|or|update|delete|\'|\/\*|\*|\.\.\/|\.\/|union|into|load_file|outfile';
if(!$value) {
        exit('没有参数！'); 
    }elseif(eregi($str, $value)) { 
        exit('参数非法！');
    } return true; 

} 
function str_check( $value ) {
   if(!get_magic_quotes_gpc()) { 
   // 进行过滤 
   $value = addslashes($value); 
   } 
   $value = str_replace("_", "\_", $value); 
  $value = str_replace("%", "\%", $value); 
   return $value; 

} 
function post_check($value) { 
        if(!get_magic_quotes_gpc()) {
    
  // 进行过滤  
            $value = addslashes($value);
        } 
        $value = str_replace("_", "\_", $value); 
        $value = str_replace("%", "\%", $value); 
        $value = nl2br($value); 
        $value = htmlspecialchars($value); 
        return $value; 
    }