Laravel, default value ignore with variable - php

Question

// EDIT
It was a problem in my controller and it specific in my project. My bad, sorry
// END EDIT
I'm pretty lost with something and I need help, this project was not done by me, I just need to do a quick fix.
I have a select in a form :
{{ Form::label('option', 'Type de contrat') }}
{{ Form::select('option[]', $contracts->lists('option', 'id')->toArray(), old('option')[0], ['class' => 'form-control selectpicker']) }}
When my form is not fill correctly, I'm redirect to the form page and I want the select have the previous value. It should be work with the "old".
But it select the first value.
So I tried var_dump(old('option')[0]) which return me "16" (string variable) who is the id of the previous selected and the value of the option who need to be selected.
But if I try (replace the variable by hardcoded value):
{{ Form::label('option', 'Type de contrat') }}
{{ Form::select('option[]', $contracts->lists('option', 'id')->toArray(), 16, ['class' => 'form-control selectpicker']) }}
It's work and I don't understand why ...
I try :
(int)old('option')[0]
But doesn't work
Laravel 5.2
Why it's work with a hardcoded int but now with a variable who return a int number ?
Select generate :
<select class="form-control selectpicker" name="option[]" tabindex="-98">
<option value="13">CDD</option><option value="14">CDI</option>
<option value="15">Stage</option>
<option value="16">Apprentissage</option>
<option value="17">Freelance</option>
</select>

Answer 1

Since it's not a multi-select, you don't need the [] in the name of the input,
You need to use it like this:
{{ Form::select('option', $contracts->lists('option', 'id')->toArray(), old('option'), ['class' => 'form-control selectpicker']) }}

Answer 2

If you have more than one, then you can try it:
view
#for($i = 1; $i <= 2; $i++)
<input type="hidden" name="count_select[]">
<select class="form-control selectpicker" name="option{$i}" tabindex="-98">
<option value="13"{{ (old("option{$i}") == 13) ? ' selected' : '' }}>CDD</option>
<option value="14"{{ (old("option{$i}") == 14) ? ' selected' : '' }}>CDI</option>
<option value="15"{{ (old("option{$i}") == 15) ? ' selected' : '' }}>Stage</option>
<option value="16"{{ (old("option{$i}") == 16) ? ' selected' : '' }}>Apprentissage</option>
<option value="17"{{ (old("option{$i}") == 17) ? ' selected' : '' }}>Freelance</option>
</select>
#endfor
controller
$count_select = $request->input('count_select');
if(count($count_select) > 0){
for($a = 1; $a <= count($count_select); $a++){
$this->validate($request, [
"option{$a}" => 'required'
]);
}
}