src/Agent.php
public function addCustomParameter(string $key, $value): bool
    {
        if (!$this->isLoaded()) {
            return false;
        }

        return newrelic_add_custom_parameter($key, $value);
    }
public function addCustomParameter(string $key, $value): bool
    {
        if (!$this->isLoaded()) {
            return false;
        }

        return newrelic_add_custom_parameter($key, $value);
    }
Attaches a custom attribute (key/value pair) to the current transaction.

Agent version 4.4.5.35 or higher.

@see https://docs.newrelic.com/docs/agents/php-agent/php-agent-api/newrelic_add_custom_parameter

@param string $key   The name of the custom attribute. Only the first 255 characters are retained
@param mixed  $value The value to associate with this custom attribute

@return bool