src/AbstractConfig.php
public function getVariable(string $name, $default = null)
    {
        return $this->defaultVariables[$name] ??
               $this->userDefinedVariables[$name] ??
               $default;
    }
public function getVariable(string $name, $default = null)
    {
        return $this->defaultVariables[$name] ??
               $this->userDefinedVariables[$name] ??
               $default;
    }
@inheritdoc

Some variables are already defined:
- php_version
- php_version_id
- php_major_version
- php_minor_version
- php_release_version
- php_sapi
- system_os
- system_os_family