gestire stringa in formato JSON

[cosmo98]

Magari se dici gli errori che ottieni si capisce qualcosa.

ho provato cosi:

    foreach ($prodotti as $idprodotto => $id) {
      foreach ($quantita as  $qta) {
        echo  $id;
        echo  $qta;
      }
    }

questi sono gli errori che mi restituisce:
Notice: Undefined variable: quantita in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Warning: Invalid argument supplied for foreach() in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Notice: Undefined variable: quantita in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Warning: Invalid argument supplied for foreach() in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Notice: Undefined variable: quantita in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Warning: Invalid argument supplied for foreach() in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Notice: Undefined variable: quantita in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Warning: Invalid argument supplied for foreach() in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Notice: Undefined variable: quantita in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Warning: Invalid argument supplied for foreach() in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Notice: Undefined variable: quantita in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45

Warning: Invalid argument supplied for foreach() in C:\Users\Cosmo\Documents\ceramica_ultimo_sito\ques toeilcarrello.php on line 45