What are the backticks `` called?

If you're referring to Bash then the backticks are known as "command substitution". $() provides similar functionality.

Backticks (``) is an execution operator. PHP will attempt to execute the contents of the backticks as a shell command; the output will be returned (i.e., it won't simply be dumped to output; it can be assigned to a variable). Use of the backtick operator is identical to shell_exec().

For example,

<?php
    $output = `ls -la`;
    echo "<pre>$output</pre>";
?>

For more information, refer to Execution Operators.