Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10 Notice: Array to string conversion in D:\xampp\htdocs\o...

Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10

Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10

Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10

Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10

Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10

Notice: Array to string conversion in D:\xampp\htdocs\ordenamiento.php on line 10

<?php

$palabras = array("sol","luna","cielo", "luz", "estrellas", "lluvia");
//$desordenadas = array();

$form ="<form action='analisis.php'>";
for ($i=0; $i < count($palabras); $i++){
  //  $desordenadas[$i] = str_shuffle($palabras[$i]);
 $form .="La palabra: ".str_shuffle($palabras[$i])." ".
 "<input type ='text' name ='palabra".[$i]."'>".
 "<br>";
}

$button ="<button type = 'submit' > Enviar</button>";
$formCierre ="</form>";
//print_r($desordenadas);

/* echo "
<form action='analisis.php'>
    <input type ='text' name ='palabra0'>
    <input type ='text' name ='palabra1'>
    <input type ='text' name ='palabra2'>
    <button type = 'submit' > Enviar</button>
</form>
"; */

echo $form.$button.$formCierre;

?>

Por que me aparece esa precaución