placeholder.php

<?php

/**
 * Interpolates context values into the message placeholders.
 */
function interpolate($message, array $context = array())
{
    // build a replacement array with braces around the context keys
    $replace = array();
    foreach ($context as $key => $val) {
        // check that the value can be cast to string
        if (!is_array($val) && (!is_object($val) || method_exists($val, '__toString'))) {
            $replace['{' . $key . '}'] = $val;
        }
    }

    // interpolate replacement values into the message and return
    return strtr($message, $replace);
}

// a message with brace-delimited placeholder names
$message = "User {username} created";

// a context array of placeholder names => replacement values
$context = array('username' => 'bolivar');

// echoes "User bolivar created"
echo interpolate($message, $context);