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.