To post an array of undefined length in HTML form element. We need different input element of form with a similar name. These elements will have square brackets in the end.
Square brackets make them an array when the form is posted.
eg:- <input type=”text” name=”key_name[]“>
<form>
<input type="text" name="key_name[]">
<input type="text" name="key_name[]">
<input type="text" name="key_name[]">
<input type="text" name="key_name[]">
<input type="submit" name="submit">
</form>
The above form will return an array of length 4, with the element in the order of declaration.
After the form is Submitted the fields with a name like “key_name[]” will be converted into a numeric array named key_name.