I am new in Expression engine. I know that We can not write PHP code in template due to security reason. I want to know the flow like from where can write php code and how it will render in my template ?
Which version of EE are you working with?
EE allows use of PHP in templates, but there are security factors to be aware of before you enable it, so you have to enable PHP processing on a per-template basis. PHP should not be a problem provided you have a good control of who can edit templates in your EE setup - vis:
Enabling PHP in a template will enable anyone with editing rights for that template to become a de-facto Super Admin since they can execute any PHP they want in that template
You can find out more about how to embed PHP in EE templates from the EE documentation