How to prevent SQL injection in PHP+Mysql

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

/**
* 防止sql注入自定义方法一
* author: xiaochuan
* @param: mixed $value 参数值
*/  
function check_param($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; 
} 
  
 
 
 
 
 
 
/**
* 防止sql注入自定义方法二
* author: xiaochuan
* @param: mixed $value 参数值
*/ 
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; 
} 
  
 
 
 
 
 
/**
* 防止sql注入自定义方法三
* author: xiaochuan
* @param: mixed $value 参数值
*/ 
function post_check($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; 
}