php语法如何将字符串强制转为数字类型

<?php
header("Content-type:text/html;charset=utf-8");
$str = '123abc456';
echo $str."<br>";
settype($str,"integer");
echo $str."<br>";
echo '修改后的类型为：' . gettype($str) . '<br>';
echo '<hr>';
 
$str = '123.456.789';
echo $str."<br>";
settype($str,"integer");
echo $str."<br>";
echo '修改后的类型为：' . gettype($str) . '<br>';
?>