Foreach循环工作，但输出“无效参数”？

Question

I hope to give enough information here, but I am confused as to why the foreach loop works, it gets each data and outputs it in an li but I am getting an invalid argument error?

Here is the result of the var_dump

array(1) { ["questions"]=> 
    array(2) { ["title"]=> string(5) "Keanu" [1]=> 
        array(1) { ["questionlist"]=> array(2) { [0]=> 
            array(1) { 
                ["a-question"]=> string(1) "1" } [1]=> array(1) { 
                ["a-question"]=> string(5) "civil" } } } } } 

Here is my foreach statement

foreach($questions['questions'] as $key => $value){    
            foreach($value['questionlist'] as $key => $subquestion){ //line 119

                 echo '<li>'.$subquestion['a-question'].'</li>';

            }
}

$questions is a variable used to get the data from the database like this.

$questions = $wpdb->get_row("SELECT * FROM $table_name ORDER BY id DESC LIMIT 1" , ARRAY_A);

The data comes from ajax, I modify the ajax $_POST like this before sending to the database.

    // Add modifications
    $questions['questions'] = $_POST['questions']['data'];

    // DB data
    $name = $wpdb->escape($questions['questions']['title']);
    $data = $wpdb->escape(json_encode($questions));

Screenshot:

I am not sure why I am getting the invalid argument, I suspect its because the array may not be formatted properly, if you need any more information let me know.

A Solution: Thanks to @didierc

I used his code and modified it a bit to display my data in a loop, basically all I did was add another foreach.

 foreach($questions['questions'] as $key => $value){   
   if(is_array($value) && isset($value[ 'questionlist'])){
       foreach($value as $key => $subquestion){ //line 119
           foreach ($subquestion as $key => $value){

               // This loops all the ['a-question'] data
               echo '<li>''.$value['a-question'].''</li>';

           }
        }
     }
  }  

Answer 1

Try this:

foreach ($questions['questions'] as $key => $value) {   
        if (is_array($value) && isset($value[ 'questionlist']))  {
            foreach ($value['questionlist'] as $subnum => $subquestion) {
                foreach ($subquestion as $qtitle => $qanswer) {

With variable names hopefully explicit enough. That should get you started.

NB: The data is probably easier to understand when formatted as below:

array(1) {
 ["questions"]=> array(2) { 
    ["title"] => string(5) "Keanu"
    [1]      =>  array(1) { 
        ["questionlist"]=> array(2) {
            [0]=>   array(1) { 
                ["a-question"]=> string(1) "1" 
            }
            [1]=> array(1) { 
                ["a-question"]=> string(5) "civil"
            }
        } 
    }
}
}