$username = $_POST['username'];
$password = $_POST['password'];
$confirm_password = $_POST['confirm_password'];

$password_filter1 = '/(?=.*[a-zA-Z0-9]{6})/m';  //Password contains more then 6 char
$password_filter2 = '/(?=.*[A-Z])/m';           //Capital letter
$password_filter3 = '/(?=.*d)/m';              //A digit

if (!empty($password & $confirm_password)) {
 if ($password === $confirm_password) {
  if (preg_match($password_filter1, $password)) {
   if (preg_match($password_filter2, $password)) {
    if (preg_match($password_filter3, $password)) {
     //If everything is true do this
    } else {
     $password_error = "Your password must contain at least 1 number";
    }
   } else {
    $password_error = "You're password must have at least 1 capital letter";
   }
  } else {
   $password_error = "You're password must be at least 6 characters long";
  }
 } else {
  $password_error = "The passwords do not match";
 }
} else {
 $password_error = "You must enter a password";
}

$password_filter1 = '/(?=.*[a-zA-Z0-9]{6})/m';  //Password contains more then 6 char
$password_filter2 = '/(?=.*[A-Z])/m';           //Capital letter
$password_filter3 = '/(?=.*d)/m';              //A digit

$has_at_least_6_chars = '/(?=.*[a-zA-Z0-9]{6})/';
$has_a_capital_letter = '/(?=.*[A-Z])/';
$has_a_digit = '/(?=.*d)/';

function has_a_capital_letter($str) {
    return preg_match('/(?=.*[A-Z])/', $str);
}

//...

if (!has_a_capital_letter($password)) {
    $password_error = "Your password must have at least 1 capital letter";
}

if (empty($password & $confirm_password)) {
    $password_error = "You must enter a password";
}
elseif ($password !== $confirm_password) {
    $password_error = "The passwords do not match";
}
elseif (!preg_match($has_at_least_6_chars, $password)) {
    $password_error = "Your password must be at least 6 characters long";
}
elseif (!preg_match($has_a_capital_letter, $password)) {
    $password_error = "Your password must have at least 1 capital letter";
}
elseif (!preg_match($has_a_digit, $password)) {
    $password_error = "Your password must contain at least 1 number";
}
else {
    //If everything is true do this
}

$password = "Hell0!";
$confirm_password = "Hell0!";

elseif (strlen($password) < 6) {
    $password_error = "Your password must be at least 6 characters long";
}

function getPasswordError($password, $confirm_password)
{
    $password_filter1 = '/(?=.*[A-Z])/m';
    $password_filter2 = '/(?=.*d)/m';

    if (empty($password) || empty($confirm_password)) {
        return 'You must enter a password';
    }

    if (strlen($password) < 6) {
        return 'Your password must be at least 6 characters long';
    }

    if ($password !== $confirm_password) {
        return 'The passwords do not match';
    }

    if (!preg_match($password_filter1, $password)) {
        return 'Your password must have at least 1 capital letter';
    }

    if (!preg_match($password_filter2, $password)) {
        return 'Your password must contain at least 1 number';
    }

    return false;
}

$password_error = getPasswordError($password, $confirm_password);

if ($password_error !== false) {
    echo $password_error; // or do whatever with the data
}