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Link para baixar a planilha

<!DOCTYPE html>
<html lang="pt-br">
	<head>
		<meta charset="utf-8">
		<title>Contato</title>
	<head>
	<body>
		<a href="gerar_planilha.php">Gerar Planilha</a>
	</body>
</html>

Função para gerar a planilha

 <!DOCTYPE html>
<html lang="pt-br">
	<head>
		<meta charset="utf-8">
		<title>Contato</title>
	<head>
	<body>
		<?php
		// Definimos o nome do arquivo que será exportado
		$arquivo = 'msgcontatos.xls';
		// Criamos uma tabela HTML com o formato da planilha
		$html = '';
		$html .= '<table border="1">';
		$html .= '<tr>';
		$html .= '<td colspan="5">Planilha Mensagem de Contatos</td>';
		$html .= '</tr>';

		$html .= '<tr>';
		$html .= '<td><b>ID</b></td>';
		$html .= '<td><b>Nome</b></td>';
		$html .= '<td><b>E-mail</b></td>';
		$html .= '<td><b>Assunto</b></td>';
		$html .= '<td><b>Data</b></td>';
		$html .= '</tr>';

		$html .= '<tr>';
		$html .= '<td>1</td>';
		$html .= '<td>Fulano</td>';
		$html .= '<td>fulano@hotmail.com.br</td>';
		$html .= '<td>Assunto do contato</td>';
		$html .= '<td>25/05/2017</td>';
		$html .= '</tr>';

		$html .= '<tr>';
		$html .= '<td>1</td>';
		$html .= '<td>Kelly</td>';
		$html .= '<td>kelly@gmail.com.br</td>';
		$html .= '<td>Assunto do contato</td>';
		$html .= '<td>25/05/2017</td>';
		$html .= '</tr>';

		$html .= '<tr>';
		$html .= '<td>1</td>';
		$html .= '<td>Jessica</td>';
		$html .= '<td>jessica@yahoo.com.br</td>';
		$html .= '<td>Assunto do contato</td>';
		$html .= '<td>25/05/2017</td>';
		$html .= '</tr>';

		$html .= '</table>';

		// Configurações header para forçar o download
		header ("Expires: Mon, 07 Jul 2016 05:00:00 GMT");
		header ("Last-Modified: " . gmdate("D,d M YH:i:s") . " GMT");
		header ("Cache-Control: no-cache, must-revalidate");
		header ("Pragma: no-cache");
		header ("Content-type: application/x-msexcel");
		header ("Content-Disposition: attachment; filename=\"{$arquivo}\"" );
		header ("Content-Description: PHP Generated Data" );
		// Envia o conteúdo do arquivo
		echo $html;
		exit;?>
	</body>
</html>