plugin_loop.php

<?php
$attributes_str = "small_image";

function IsNullOrEmptyString($value){
    return (!isset($value) || trim($value)==='');
}

foreach (explode(",", $attributes_str) as $attr) {
    $meta_tag = '<meta name="product:%s" content="%s" />';
    $value = $product->getData();

    if (!IsNullOrEmptyString($value)) {
        echo sprintf($meta_tag, $attr, $product->getAttributeText($attr));
    }
}
?>