preprocessor for arbitrary text

preprocessor.php

<?php

$config = ...;
$input = file_get_contents('php://stdin');
$output = $input;
$opening = '<%';
$closing = '%>';

for ($offset = strpos($output, $opening); $offset !== false; $offset = strpos($output, $opening)) {
    $endOffset = strpos($output, $closing, $offset);
    $expression = substr($output, $offset + strlen($opening), $endOffset - $offset - strlen($opening));
    $expression = trim($expression);
    $output = substr_replace($output, evaluate($expression, $config), $offset, $endOffset - $offset + 2);
}

function evaluate($expression, $config)
{
    $value = '';
    $command = $expression[0];
    $inner = trim(substr($expression, 1));
    if ($command === '=') {
        $chunks = explode('.', $inner);
        $value = $config;

        foreach ($chunks as $chunk) {
            $value = $value[$chunk];
        }
    } else if ($command === '.') {
        return file_get_contents($inner);
    }
    return $value;
}

echo $output;