Wu Yuxiong - natural born PHP development of learning: string variable

<?php 
$txt="Hello world!"; 
echo $txt; 
?>

<?php
$txt1="Hello world!";
$txt2="What a nice day!";
echo $txt1 . " " . $txt2;
?>

<?php
echo strlen("Hello world!");
?>

<?php
echo strpos("Hello world!","world");
?>

To be perfect ....