If you need to do lead generation, you can skip the use of API if you don't want it.
To do so, without API, you’ll have to download the toolkit to connect it with Salesforce from your PHP and you will get below files in the toolkit:
1- SforceBaseClient.php
2- SforceHeaderOptions.php
3- SforcePartnerClient.php
4- partner.wsdl.xml
After downloading, you'll also get an example file which would be a good example for you to connect and post your data.