dependent and conditional validations not working - php

Question

Hi all am trying to do validations for a text field which is depends on other selected value .there is two columns buyer and reference number.so the validation is when i select particular names in buyer column then the reference must be required.
Below is my code
model::
public static $rules = array(
'delivery_contact_no' => 'required|numeric',
'external_ref_number' => 'required',
);
controller::
$check = false;
if ($_POST["user"] == '11Street') {
$check = true;
if (Input::get('external_ref_number') == "") {
$rules = array(
'external_ref_number' => 'required'
);
$messages = array(
'external_ref_number.required' => 'The External Reference Number Must be required ',
);
}
if ($check) {
$validator = Validator::make(Input::all(), $message);
if ($validator->fails()) {
return Redirect::back()->withInput()->withErrors($validator);
}
}
}
I am inserting input data like below:
$get = array(
'user' => trim($_POST["user"]),
'external_ref_number' => trim(Input::get('external_ref_number')),
);
so the validation is when I select particular names in buyer column then the reference must be required. below is view code
view::
<div class="form-group #if ($errors->has('user')) has-error #endif">
{{ Form::label('user', 'Buyer *', array('class'=> 'col-lg-2 control-label')) }}
<div class="col-lg-4">
{{ Form::text('user', Input::old('user'), array('class'=> 'form-control', 'autofocus' => 'autofocus', 'readonly' => 'readonly')) }}
{{ $errors->first('user', '<p class="help-block">:message</p>') }}
<input type="hidden" id="user_id" name="user_id">
</div>
<div class="col-xs-1">
<div class="input-group">
<div class="input-group-btn">
<span class="pull-left"><button id="selectUserBtn" class="btn btn-primary selectUserBtn" href="/transaction/ajaxcustomer"><i class="fa fa-plus"></i> Select Buyer</span>
</div>
</div>
</div>
</div>
<div class="form-group required {{ $errors->first('external_ref_number', 'has-error') }}">
{{ Form::label('external_ref_number', 'External reference number ', array('class' => 'col-lg-2 control-label')) }}
<div class="col-lg-4">
{{ Form::text('external_ref_number', $orderMappedInfo['external_ref_number'], ['placeholder' => 'External reference number', 'class' => 'form-control']) }}
{{ $errors->first('external_ref_number', '<p class="help-block">:message</p>') }}
</div>
</div>
The validation code which I wrote is not working. Please help me to solve this issue.
I am trying to do validations for a text field which is depends on other selected value.
There are two columns buyer and reference number.
so the validation is when i select particular names in buyer column then the reference must be required.