Formats a string using an associative array.

nsprintf.php

/**
 * Formats a string using an associative array.
 * Example:
 * echo nsprintf(":name :surname is :age", array('name' => 'John', 'age' => '29', 'surname' => 'Doe'));
 *
 * @param string $format A formatted string with named template fields
 * @param array $args An associative array of values to place in the formatted string.
 * @return string A formatted string
 */
function nsprintf($format, $args)
{
    $names = preg_match_all('/\:\S*\b/', $format, $matches, PREG_SET_ORDER);

    $values = array();
    foreach ($matches as $match) {
        $values[] = $args[str_replace(':', '', $match[0])];
    }

    $format = preg_replace('/\:\S*\b/', '%s', $format);
    return vsprintf($format, $values);
}

Sweet ;)

See my implementation - https://github.com/Cosmologist/Gears/blob/master/src/Gears/StringType.php#L115
You can specify another modificators (not %s only) and use more standard expression style - like 'test %(string)s'