"Illegal string offset" error when calling usort in database data

Question

I am trying to sort the result by a specific column of a database array.

 function cmp($a, $b) {
 return strcmp($a['usu_codigo'], $b['usu_codigo']);
  };

usort($usu_info_coluna, "cmp");

Giving a var_dump in $usu_info_Coluna, he returns:

array(21) { ["usu_codigo"]=> int(1) ["usu_nome"]=> string(5) "LUANN" ["usu_senha"]=> string(60) "$2y$10$I4RqdKD/cOwRDNpFgtIbWeVirNIfxHPREMEklGaBuONGRZMtQfUgq" ["usu_sobrenome"]=> string(5) "SOUSA" ["usu_cpf"]=> string(11) "12345678998" ["usu_rg"]=> string(9) "123456789" ["usu_nasc"]=> string(10) "2018-10-22" ["usu_endereco"]=> string(4) "AV 2" ["usu_numero"]=> string(2) "97" ["usu_bairro"]=> string(8) "BLOCO D2" ["usu_cep"]=> string(8) "11900000" ["usu_cidade"]=> string(11) "REGISTRO-SP" ["usu_uf"]=> string(2) "SP" ["usu_tel"]=> string(10) "1338226293" ["usu_cel"]=> string(11) "13997821923" ["usu_genero"]=> string(9) "Masculino" ["usu_situacao"]=> string(5) "ativo" ["usu_email"]=> string(22) "[email protected]" ["usu_indicador_codigo"]=> NULL ["usu_datacadastro"]=> string(10) "2018-10-22" ["usu_nivel"]=> string(3) "adm" } 

What did I do wrong using usort? I changed and changed the code of the usort, but always returns me a different error, and I believe that was as close as I got.

UPDATE 0: This is the prepared query I use for the query:

$_SESSION['codigo'] = 1; 
$usu_codigo = $_SESSION['codigo'];
$usu_situacao = 'ativo';

$stmt3 = $conexao->prepare('SELECT * FROM esc_usuarios WHERE usu_indicador_codigo = ?');
$stmt3->bind_param('i', $usu_codigo);
$stmt3->execute();
$usu_ult5_cad = $stmt3->get_result();

Displaying through a table:

<?php

$limit = 2;
while($limit -- && $coluna_ult5 = $usu_ult5_cad->fetch_array()){

    ?>
    <tbody>
    <tr>
      <th><?php echo $coluna_ult5['usu_codigo']; ?><br></th>
      <td><?php echo $coluna_ult5['usu_nome']; ?></td>
      <td><?php echo $coluna_ult5['usu_sobrenome']; ?><br></td>
    </tr>                                               
<?php }

?>

Answer 1

The first step of everything is to try to translate the error to know what it is.

Illegal string offset is returned when you try to get a offset of a string, passing a string per parameter.

Behold:

$a = 'wallace';

var_dump($a[0]); // string 'w'

var_dump($a['string']); // Erro: Illegal string offset 'string' on line 1

See the test on ideone.

That’s because the operator [] works so much to array, as for string in PHP. In the case of string you can get the character according to the position passed.

However, if you pass a string, it causes an error. This is something PHP allows for strings and few people know.

What’s happening within your function is basically this.

When you pass your array for usort, He’ll pick up the items from his array of 2 and 2 and pass them by callback parameter of usort.

Exemplifying the case of your array, he’s like this:

  ['a' => 1, 'b' => 2, 'c' => 3]

If you pass this array for usort, the callback would place 1 in $a and 2 in $b.

And so it is giving error Illegal string offset. Because you assumed that inside the callback was passing one array, but is actually passing strings or integers.

So it can work out the way you’re trying to, your array should be like this:

  [
        ['usu_codigo' => 1, /** resto do array **/],
        ['usu_codigo'   => 2, /** resto do array **/],
        ['usu_codigo'   => 3, /** resto do array **/],
  ]

See an example on ideone

Answer 2

Edit: As corrected in the comments, the error "Illegal string offset" happens when you try to access the index of a string using another string (as explained best in the other answer).

And the mistake "Undefined offset" (that I had mistakenly confused before) happens when you try to access an index that does not exist in the array. In this error it is possible to use "isset" to check and test the array value before performing any operation.