Impossible to convert json to php array

Your json is malformed (you can see json decode errors using json_last_error(), as explained on the official documentation: json_last_error) due to missing quotes.

Try the following json string:

$mce_settings = '{"alignleft":[{"selector":"p,h1,h2,h3,h4,h5,h6,td,th,div,ul,ol,li","styles":{"textAlign":"left"}},{"selector":"img,table,dl.wp-caption","classes":"alignleft"}],"aligncenter":[{"selector":"p,h1,h2,h3,h4,h5,h6,td,th,div,ul,ol,li","styles":{"textAlign":"center"}},{"selector":"img,table,dl.wp-caption","classes":"aligncenter"}],"alignright":[{"selector":"p,h1,h2,h3,h4,h5,h6,td,th,div,ul,ol,li","styles":{"textAlign":"right"}},{"selector":"img,table,dl.wp-caption","classes":"alignright"}],"strikethrough":{"inline":"del"}}';

var_dump(json_decode( $mce_settings, true ));

This prints:

array(4) {
  ["alignleft"]=>
  array(2) {
    [0]=>
    array(2) {
      ["selector"]=>
      string(38) "p,h1,h2,h3,h4,h5,h6,td,th,div,ul,ol,li"
      ["styles"]=>
      array(1) {
        ["textAlign"]=>
        string(4) "left"
      }
    }
    [1]=>
    array(2) {
      ["selector"]=>
      string(23) "img,table,dl.wp-caption"
      ["classes"]=>
      string(9) "alignleft"
    }
  }
  ["aligncenter"]=>
  array(2) {
    [0]=>
    array(2) {
      ["selector"]=>
      string(38) "p,h1,h2,h3,h4,h5,h6,td,th,div,ul,ol,li"
      ["styles"]=>
      array(1) {
        ["textAlign"]=>
        string(6) "center"
      }
    }
    [1]=>
    array(2) {
      ["selector"]=>
      string(23) "img,table,dl.wp-caption"
      ["classes"]=>
      string(11) "aligncenter"
    }
  }
  ["alignright"]=>
  array(2) {
    [0]=>
    array(2) {
      ["selector"]=>
      string(38) "p,h1,h2,h3,h4,h5,h6,td,th,div,ul,ol,li"
      ["styles"]=>
      array(1) {
        ["textAlign"]=>
        string(5) "right"
      }
    }
    [1]=>
    array(2) {
      ["selector"]=>
      string(23) "img,table,dl.wp-caption"
      ["classes"]=>
      string(10) "alignright"
    }
  }
  ["strikethrough"]=>
  array(1) {
    ["inline"]=>
    string(3) "del"
  }
}