ขอความช่วยเหลือหน่อยครับ ต้องการ save ข้อมูลจาก record ที่ค้นหาเป็นไฟล์ json ครับ

<form method="post" action="searchtest.php" accept-charset="utf-8">
		<input type="hidden" name="submitted" value="true">
		<label> Search Category : 
			<select name="category" >
				<option value="firstname">firstname</option>
				<option value="surname">surname</option>
				<option value="address">address</option>
				<option value="tel">tel</option>
				<option value="email">email</option>
				<option value="gender">gender</option>
			</select>
		</label>
		<label>Search : <input type="text" name="search"></label>
		<input type="submit" name="">
	</form>
	<?php
		if (isset($_POST['submitted'])) {	
			include('connectdb.php');
			$category = $_POST['category'];
			$search = $_POST['search'];
			$query = "SELECT * FROM users WHERE $category LIKE '%".$search."%'";
			$result = mysqli_query($conn, $query) or die ('error');
			$user_data = array();
			$num_rows = mysqli_num_rows($result);
			echo "$num_rows result found";
			echo "<table>";
			echo "<tr><th>firstname</th><th>surname</th><th>address</th><th>tel</th><th>email</th><th style='text-align:right'>gender</th></tr>";
			while ($row = mysqli_fetch_array($result, MYSQLI_ASSOC)){
				echo"<tr><td>";
				echo $row['firstname'];
				echo "</td><td>";
				echo $row['surname'];
				echo "</td><td>";
				echo $row['address'];
				echo "</td><td>";
				echo $row['tel'];
				echo "</td><td>";
				echo $row['email'];
				echo "</td><td style='text-align:right'>";
				echo $row['gender'];
				echo "</td></tr>";

				$user_data[] = array(
					'firstname'	=>	$row["firstname"],
					'surname'	=>	$row["surname"],
					'address'	=>	$row["address"],
					'tel'	=>	$row["tel"],
					'email'	=>	$row["email"],
					'gender'	=>	$row["gender"]
				);

			}
			echo "</table>";
			
			
		}
	?>