PHP未定义的索引/变量

第一行出现错误未定义索引：$pwd下面还有未定义的变量。请出示新代码谢谢！结局唾手可得。

$pwd = $_POST['$pwd'];              //Error undefined index: $pwd
if( strlen($pwd) < 8 ) {
        $error .= "Password too short! 
";
}                                     //This line undefined variable

在此处获取上面的错误******错误变量

if( strlen($pwd) > 20 ) {
        $error .= "Password too long! 
";
}
if( strlen($pwd) < 8 ) {
        $error .= "Password too short! 
";
}
if( !preg_match("#[0-9]+#", $pwd) ) {
        $error .= "Password must include at least one number! 
";
}

if( !preg_match("#[a-z]+#", $pwd) ) {
        $error .= "Password must include at least one letter! 
";
}

if( !preg_match("#[A-Z]+#", $pwd) ) {
        $error .= "Password must include at least one CAPS! 
";
}

if( !preg_match("#'W+#", $pwd) ) {
        $error .= "Password must include at least one symbol! 
";
}

if($error){
        echo "Password validation failure(your choice is weak): $error";
} else {
        echo "Your password is strong.";
}

再次感谢：D