Converted 0->feild0, 1->field1,2->field2.... 
This is just one example in which i get comma separated value in string and convert it into multidimensional array and then using foreach loop i changed key value of array  
<?php
    $str = "abc,def,ghi,jkl,mno,pqr,stu 
    abc,def,ghi,jkl,mno,pqr,stu
    abc,def,ghi,jkl,mno,pqr,stu
    abc,def,ghi,jkl,mno,pqr,stu;
    echo '<pre>';
    $arr1 = explode("\n", $str); // this will create multidimensional array from upper string
    //print_r($arr1);
    foreach ($arr1 as $key => $value) {
        $arr2[] = explode(",", $value);
        foreach ($arr2 as $key1 => $value1) {
            $i =0;
            foreach ($value1 as $key2 => $value2) { 
                $key3 = 'field'.$i;
                $i++;
                $value1[$key3] = $value2;
                unset($value1[$key2]);           
            }
        }
        $arr3[] = $value1;
    }
    print_r($arr3);
   ?>