One way is via a link:
<?php
// file1.php
$var1 = 'Here is my var';
?>
<a href="file2.php?my_var=<?php echo urlencode($var1) ?>">A link</a>
<?php
// file2.php
echo $_GET['my_var'];
?>
Another way is via session:
One way is via a link:
<?php
// file 3
session_start();
$_SESSION['my_var'] = 'Here is my var!';
?>
<?php
// file 4
session_start();
echo $_SESSION['my_var'];
?>