laravel 4输入数组,带质量分配

I have this array input:

<input type="text" class="form-control"  name="item[]">

<input type="text" class="form-control"  name="item[]">

that can be found more then once within the form.

and i try to use mass assignment but i end up with error:

preg_replace(): Parameter mismatch, pattern is a string while replacement is an array

on top of that.. Before i do the mass assignment i want to turn the input array into json so i could store it within one column in the table.