A Google Script function that can be used to populate dependent dropdowns

GetPhiAreas.gs

function onEdit(e) {
  var ss = SpreadsheetApp.getActive();
  var sampleDataSheet = ss.getSheetByName("sample_data");
  var mohPhiDataSheet = ss.getSheetByName("moh_phi_data");

  // Check if the active sheet is "sample_data" and the active cell is in the first column (MOH area)
  if (ss.getActiveSheet().getName() == "sample_data" && e.range.getColumn() == 1) {
    // Notify user of populating process
    SpreadsheetApp.getActiveSpreadsheet().toast("Populating PHI Values")

    var currRow = e.range.getRow();
    var mohArea = e.range.getValue();

    // Get the range of MOH areas and PHI areas in the "moh_phi_data" sheet
    var mohRange = mohPhiDataSheet.getRange("B:B");
    var phiRange = mohPhiDataSheet.getRange("C:C");

    var mohValues = mohRange.getValues();
    var phiValues = phiRange.getValues();

    var phiAreaValues = [];

    // Get the PHI area values that correspond to the selected MOH area
    for (var i = 0; i < mohValues.length; i++) {
      if (mohValues[i][0] == mohArea) {
        phiAreaValues.push(phiValues[i][0]);
      }
    }

    // Create data validation rule for PHI area dropdown
    var phiAreaRule = SpreadsheetApp.newDataValidation().requireValueInList(phiAreaValues).build();

    // Set the data validation rule for the PHI area cell in the current row
    sampleDataSheet.getRange(currRow, 2).setDataValidation(phiAreaRule);
  }
}