example.php

<?php

$marge = new Marge();
$marge->setData([
  'first_name' => 'David',
  'last_name' => 'Hemphill',
]);

$marge->replace("Thanks, [FIRST_NAME] [LAST_NAME] for doing yer thang!");

// Desired result
"Thanks David Hemphill for doing yer thang!";

Marge.php

<?php

namespace App\Merge;

class Marge
{
    protected $data = [];

    public function setData($data)
    {
        $this->data = $data;

        return $this;
    }

    public function getData()
    {
        return $this->data;
    }

    protected function addDelimiters($token)
    {
        return '[' . $token . ']';
    }

    protected function formatToken($token)
    {
        return $this->addDelimiters(strtoupper($token));
    }

    public function replace($text)
    {
        return collect($this->getData())->map(function ($item, $key) {
            return [$this->formatToken($key), $item];
        })->reduce(function ($text, $replacement) {
            list($search, $replace) = $replacement;
            return str_replace($search, $replace, $text);
        }, $text);
    }
}