PHP – An unidentified value inside array suddenly appear after unset()?

What Im trying to do is I want to delete specific array index value inside this array that Im comparing it with another array. I heard unset is the way to go so I give it a try. Below is the json encode before the unset().

[
 {
  "crs_id": "1269",
  "crs_code": "UI3/212",
  "crs_title_bm": "LOL",
  "crs_title_bi": "OLO",
  "crs_description_bm": "ye ah tu",
  "crs_description_bi": "righttt",
  "crs_edit_by": "991018222222",
  "crs_edit_date": "2022-04-11 05:43:30",
  "crs_aprv_type1": "0",
  "crs_aprv_val1": null,
  "crs_aprv_type2": "0",
  "crs_aprv_val2": null,
  "crs_aprv_type3": "0",
  "crs_aprv_val3": null,
  "crs_aprv_type4": "0",
  "crs_aprv_val4": null,
  "crs_aprv_type5": "0",
  "crs_aprv_val5": null,
  "crs_indakua": "1",
  "crs_indpengembangan": "0",
  "crs_status": "1",
  "cat_id": "27",
  "typ_id": "7",
  "clssfctn_id": "8",
  "crs_grp_id": "8",
  "crs_org_id": "16",
  "cdept_id": "40408000017"
 },
 {
  "crs_id": "1270",
  "crs_code": "ABC18",
  "crs_title_bm": "LOL2",
  "crs_title_bi": "OLO2",
  "crs_description_bm": "yow",
  "crs_description_bi": "right",
  "crs_edit_by": "991018222222",
  "crs_edit_date": "2022-04-11 05:42:52",
  "crs_aprv_type1": "0",
  "crs_aprv_val1": null,
  "crs_aprv_type2": "0",
  "crs_aprv_val2": null,
  "crs_aprv_type3": "0",
  "crs_aprv_val3": null,
  "crs_aprv_type4": "0",
  "crs_aprv_val4": null,
  "crs_aprv_type5": "0",
  "crs_aprv_val5": null,
  "crs_indakua": "1",
  "crs_indpengembangan": "0",
  "crs_status": "1",
  "cat_id": "27",
  "typ_id": "7",
  "clssfctn_id": "4",
  "crs_grp_id": "9",
  "crs_org_id": "11",
  "cdept_id": "40405000000"
 },
 {
  "crs_id": "1271",
  "crs_code": "TIME12",
  "crs_title_bm": "CHECK",
  "crs_title_bi": "CHECK",
  "crs_description_bm": "nk check it is mase",
  "crs_description_bi": "nk check it is mase",
  "crs_edit_by": "991018222222",
  "crs_edit_date": "2022-04-11 05:41:54",
  "crs_aprv_type1": "0",
  "crs_aprv_val1": null,
  "crs_aprv_type2": "0",
  "crs_aprv_val2": null,
  "crs_aprv_type3": "0",
  "crs_aprv_val3": null,
  "crs_aprv_type4": "0",
  "crs_aprv_val4": null,
  "crs_aprv_type5": "0",
  "crs_aprv_val5": null,
  "crs_indakua": "1",
  "crs_indpengembangan": "0",
  "crs_status": "1",
  "cat_id": "27",
  "typ_id": "4",
  "clssfctn_id": "4",
  "crs_grp_id": "3",
  "crs_org_id": "18",
  "cdept_id": "40408000034"
 }
]

It is exactly what I hope to see so nothing wrong here. Below is the json encode after the unset().

{
 "1": {
       "crs_id": "1270",
       "crs_code": "ABC18",
       "crs_title_bm": "LOL2",
       "crs_title_bi": "OLO2",
       "crs_description_bm": "yow",
       "crs_description_bi": "right",
       "crs_edit_by": "991018222222",
       "crs_edit_date": "2022-04-11 05:42:52",
       "crs_aprv_type1": "0",
       "crs_aprv_val1": null,
       "crs_aprv_type2": "0",
       "crs_aprv_val2": null,
       "crs_aprv_type3": "0",
       "crs_aprv_val3": null,
       "crs_aprv_type4": "0",
       "crs_aprv_val4": null,
       "crs_aprv_type5": "0",
       "crs_aprv_val5": null,
       "crs_indakua": "1",
       "crs_indpengembangan": "0",
       "crs_status": "1",
       "cat_id": "27",
       "typ_id": "7",
       "clssfctn_id": "4",
       "crs_grp_id": "9",
       "crs_org_id": "11",
       "cdept_id": "40405000000"
      }
 }

The above json encode is still what I hope to see, it show the correct array key index to be shown but what make it a problem for me is the value "1" suddenly appear and I noticed it no longer in an array form(?) since before unset it shown [.....] but after unset it shown {....}

What I want to see is the "1" is gone and instead of {....} I want it to be [.....]