这个数字如何输出不了

这个数字怎么输出不了？

PHP code

<!--

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-->$str = '12345678901234567890123456789012345678901234567890';

$num = $str + 1;

echo $num;

PHP code

$str = '12345678901234567890123456789012345678901234567890'; 
echo bcadd($str,'1'); <div class="clear">
                 
              
              
        
            </div>