You could try something like this:
page1.php
<form id=form1 method=post action=page1.php>
<select name='files' onchange=document.forms.form1.submit();>
<option value=file1.txt>file1
<option value=file2.txt>file2
<option value=file3.txt>file3
</select>
</form><br><br>
<?php
if(isset($_POST['files']))
{
$getfile = file_get_contents($_POST['files']);
echo "<textarea>$getfile</textarea>";
}
?>
This should work, I think ;)
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<><