- var_dump: documentation ( source)
- preg_replace: documentation ( source)
<?php
$name = 'base_url';
$value = 'new_value';
$config = '$config["base_url"] = "old_value";';
$config = preg_replace('/\[\''.$name.'\'\]\s*=\s*(\'|\")(.*)\\1;/', "['".$name."'] = '$value';", $config);
echo '<pre>', var_dump($config), '</pre>';