Can I subscript an array variable inside a double quoted PHP string?

have you tried this?

foreach ($this->cron_hooks as $name => $options) {
    echo "Hook '$name' will run every ${$options['frequency']} seconds.";    
}

Try:

${options['frequency']}

See: https://www.php.net/manual/en/language.types.string.php#language.types.string.parsing.complex