$days * 24 * 60 * 60出错提示“Warning: A non-numeric value encountered”解决方法
本文介绍php出现Warning: A non-numeric value encountered问题，用实例分析出现这种错误的原因，并提供避免及解决问题的方法。
$daysago = date( "Y-m-d H:i:s", strtotime($today) - ($days * 24 * 60 * 60) );
以上代码执行后会提示 Warning: A non-numeric value encountered
查看PHP7.1官方文档，对这种错误的解释
New E_WARNING and E_NOTICE errors have been introduced when invalid strings are coerced using operators >expecting numbers (+ - / * % << >> | & ^) or their assignment equivalents. An E_NOTICE is emitted when >the string begins with a numeric value but contains trailing non-numeric characters, and an E_WARNING is >emitted when the string does not contain a numeric value.
在使用(+ - * / ** % << >> | & ^) 运算时，例如a+b，如果a是开始一个数字值，但包含非数字字符(123a)，b不是数字值开始时(b456)，就会有A non-numeric value encountered警告。