You must connect to a bitcoin node in one way or another to send it to the newtork. There are a number of open source php and bitcoin repositories that will do this, see here https://github.com/search?q=bitcoind+php&ref=cmdform
Another option if you do not want to run the bitcoin client locally is to connect to an api to transmit your transaction.
An example that I found for the php connection to blockchain.info is the following
message; $ txid = $ json_feed-> tx_hash; ?>
I hope this helps
UPDATE: to execute requests to the blockchain.info API, you must follow the configuration instructions here https://www.blockchain.com/api/blockchain_wallet_api
NOTE To use this API, you must run a small local service that will be responsible for managing your Blockchain wallet. Your application interacts with this service locally through HTTP API calls. Click here for complete configuration instructions on GitHub.