Hide sensitive PHP script

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HTML

PHP

security

HTML5

Hey there, I'm a young coder!

I'm making a prototype website builder, and I have a PHP script that is able to create a sub-domain from my form.

Is there any way that I can prevent malicious users from, say, spamming the form? My HTML is as follows:

<form action="website-build/build.php" method="post">

This would allow people to know the location of the script and subsequently use the inputs as data.

These are my inputs (they are validated through HTML, I just didn't include the pattern value):

<input name="first_name" type="text" />
<input name="last_name" type="text" />
<input name="email" type="email" />
<input name="cms" type="radio" value="wordpress" />
<input name="cms" type="radio" value="drupal" />
<input name="cms" type="radio" value="joomla" />
<input name="subdomain" type="text" />

The URL would look like: ...build.php?first_name=VALUE&last_name=VALUEemail=VALUE&cms=VALUE&subdomain=VALUE

So how can I stop people from submitting this in the URL (GET) or with a tool (POST) and thereby allowing no access to said script unless used by my website?

I'm thinking the best course of action would be to hide the name attribute of the inputs somehow and then people wouldn't know.

What I have tried:

I thought about deleting the action and having the PHP run in the actual document, but that doesn't help, it does the same thing. I have yet to figure out how to remove the name attribute of the inputs.