Help writing to file after specific keyword in file

[djones]

I need to add these lines to my .htaccess file via PHP

 

RewriteCond %{REQUEST_URI} checkout 
RewriteRule ^(.*)$ https://myurl.com/checkout/$1 [R,L]

 

RIGHT after this line:

RewriteBase /

 

AND before this line:

RewriteCond %{REQUEST_FILENAME} !-f

 

This is how the current file looks w/o the new line addition:

 

RewriteEngine On

RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)$ index.php [L,QSA]

 

It needs to look like this when the new lines are added(extra line breaks not necessary):

 

RewriteBase /

RewriteCond %{REQUEST_URI} checkout 
RewriteRule ^(.*)$ https://myurl.com/checkout/$1 [R,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.+)$ index.php [L,QSA]

 

The line numbers may not match up, so I cannot use the line number.

[Adam]

Forgive for asking the obvious, but you can't you just manually add these? You may also run into problems with write permissions; it's not likely PHP will have access by default.

[djones]

I'm allowing the user to turn on SSL for the checkout page. I am checking if the file is writable via PHP, if it is I want PHP to add these lines, if it's not I am telling the user they need to add these lines manually.