Get Values From Google Sheets API

SheetsGet.php

<?php
$exampleEmail = 'mikerosoft@gmail.com';

$result = $this->service->spreadsheets_values->get(SHEET_ID, 'C2:C');
$emails = $result->values;

// $emails will be an array, that you can loop through
for($i = 0; $i < count($emails); $i++) {
  if($exampleEmail === $emails[$i][0]) {
    // The email exists in the spreadsheet already.
  }
}