<?php echo substr_replace("1234567890","",2) 输出string(6) "12" ?>
为空则是删除
echo substr_replace("1234567890","5",2); 输出string(4) "125"
<?php echo substr_replace("1234567890","5",-2) 输出string(6) "123456785" ?>
<?php echo substr_replace("1234567890","5",0); 输出string(1) "5" ?>
<?php echo substr_replace("1234567890","5",0,0); 输出string(11) "51234567890" ?>