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What I want to achieve: I want to use a simple PHP form to send data to a MySQL database. However, when the input contains a certain string, I want to remove it before it is saved in the MySQL database.
Example: input is 'https://www.google.com', but I want to remove the 'https://' part, so only 'www.google.com' is saved in the database.
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You can use str_replace() to remove the parts you don't want:
$input = 'https://www.google.com';
$replaced = str_replace('https://', '', $input);
echo $replaced;
If you want to replace multiple values in one pass, you also do that (this will remove http://, https:// and ftp://):
$unwanted = array("http://", "https://", "ftp://");
$replace = array("", "", "");
$replaced = str_replace($unwanted, $replace, $input);
See: https://secure.php.net/manual/en/function.str-replace.php