You can get a Developer_Key from here Get your Google Developer Key
{select as answered, if it answered.}
if you check this code reference, it states
Set DEVELOPER_KEY to the "API key" value from the "Access" tab of the Google APIs Console http://code.google.com/apis/console#access`
Wiki on step by step to get API Key & secret
http://code.garyjones.co.uk/google-developer-api-key
OR
If say, for instance, you have a web app which would require a API key then check this:
The Service application that you have created can be used by your Web apps such as PHP, Python, ..., etc.