json字符串到php数组

I have this json string as below:

$json = '[{"sessionNo":"1","sessionData":["4","6"]},{"sessionNo":"2","sessionData":["2"]},{"sessionNo":"3"}]';

I want to "translate" it as a PHP array. I tried to do the following but it returns an empty array :

var_dump(json_decode($json))

**UPDATE**

Now I am getting this:

array (size=3)
  0 => 
    array (size=2)
      'sessionNo' => string '1' (length=1)
      'sessionData' => 
        array (size=2)
          0 => string '4' (length=1)
          1 => string '6' (length=1)
  1 => 
    array (size=2)
      'sessionNo' => string '2' (length=1)
      'sessionData' => 
        array (size=1)
          0 => string '2' (length=1)
  2 => 
    array (size=1)
      'sessionNo' => string '3' (length=1)

I want to loop through this array so I get for each sessionNo the corresponding SessionData, smth like:

sessionNo   SessionData
1           4
1           6
2           2 

You can use something like...

$json = '[{"sessionNo":"1","sessionData":["4","6"]},{"sessionNo":"2","sessionData":["2"]},{"sessionNo":"3"}]';
$array = json_decode($json, true);

foreach ( $array as $session )  {
    if ( isset ($session['sessionData'])){
        foreach ( $session['sessionData'] as $data )    {
            echo $session['sessionNo']."-".$data.PHP_EOL;
        }
    }
}

This is just converting the data and then loop over the arrays in a foreach(), only doing the inner one if there is any sessionData. This outputs..

1-4
1-6
2-2