Use file pointer in insert mode, not overwrite mode in PHP

Question

I have noticed that in PHP (7.x), when you write to a file, it overwrites any existing characters. For example,

$file = fopen("test.txt", "r+");

/* test.txt contains
abc123
*/

fwrite($file, "~");

/* test.txt now contains
~bc123
*/

fclose($file);

This is a simple example - I could have stored all of the file contents, reopened in write mode, type ~, type the stored contents, done - but my file is going to become large (meaning variable size), and it contains multiple records, not just one like this example.

What I want is something like this:

$file = fopen("test.txt", "ir+"); // insert mode r+

/* test.txt contains
abc123
*/

fwrite($file, "~");

/* test.txt now contains
~abc123
*/

fclose($file);

Is there a way to do this?