This is simple example by using ternary operator to set selected=selected
<?php $plan = array('1' => 'Green','2'=>'Red' ); ?>
<select class="form-control" title="Choose Plan">
<?php foreach ($plan as $key => $value) { ?>
<option value="<?php echo $key;?>" <?php echo ($key == '2') ? ' selected="selected"' : '';?>><?php echo $value;?></option>
<?php } ?>
</select>