php怎么将字符串转为布尔类型

<?php
    header("Content-type:text/html;charset=utf-8");
    $str = "123abc";
    echo &#39;变量的原类型为：&#39;;
	var_dump($str);
    $bool = (bool)$str;
    echo &#39;变量转换后的类型为：&#39;;
	var_dump($bool);
?>

<?php
    header("Content-type:text/html;charset=utf-8");
    $str = "hello";
    echo &#39;变量的原类型为：&#39;;
	var_dump($str);
    $bool = boolval($str);
    echo &#39;变量转换后的类型为：&#39;;
	var_dump($bool);
?>

<?php
    header("Content-type:text/html;charset=utf-8");
    $str = "hello";
    echo &#39;变量的原类型为：&#39;;
    var_dump($str);
    $bool = settype($str,"boolean");
    echo &#39;变量转换后的类型为：&#39;;
    var_dump($str);
?>