PHP字符串到整数转换不使用此代码

Question

在将字符串转换为整数之前一切正常.转换后,该值变为0.为什么？

<script type="text/javascript">
    var offset = new Date().getTimezoneOffset(); //get client timezone differance with UTC
    offset *= -1; // change sign
    offset *= 60; // convert into second
    console.log(offset);
</script>

<?php 
    echo "<br/><br/>";
    $a = "<script>document.write(offset)</script>"; //getting value in string
    echo $a; // this works and print : 19800
    echo "<br/>"; 
    settype($a, "integer");
    echo $a; // print 0
?>