PS: see /wp-content/plugins/advanced-custom-fields-pro/fields/select.php
At line 349:
$this->walk( $field['choices'], $field['value'] );
Then line 388:
$this->walk( $v, $values );
Which ends at 401:
// vars
$search = html_entity_decode($k);
$pos = array_search($search, $values);
$atts = array( 'value' => $k );
// validate selected
if( $pos !== false ) {
$atts['selected'] = 'selected';
$atts['data-i'] = $pos;
}