php heredoc deneyebilirsin. $icerik = <<<END
<tbody>
<tr>
<td align="center" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative">
<table width="100%" cellpadding="0" cellspacing="0" role="presentation" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;-premailer-cellpadding: 0;-premailer-cellspacing: 0;-premailer-width: 100%;margin: 0;padding: 0;width: 100%">
<tbody>
<tr>
<td style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;padding: 25px 0;text-align: center">
<a href="https://meetweb.me/" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;color: #3d4852;font-size: 19px;font-weight: bold;text-decoration: none;display: inline-block" target="_blank" tabindex="-1" rel="external" class="external"><img style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;max-width: 100%;border: none" src="https://meetweb.me/features/assets/images/logo.svg"></a>
</td>
</tr>
<tr>
<td width="100%" cellpadding="0" cellspacing="0" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;-premailer-cellpadding: 0;-premailer-cellspacing: 0;-premailer-width: 100%;background-color: #e8e6fa;border-bottom: 1px solid #e8e6fa;border-top: 1px solid #e8e6fa;margin: 0;padding: 0;width: 100%">
<table align="center" width="570" cellpadding="0" cellspacing="0" role="presentation" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;-premailer-cellpadding: 0;-premailer-cellspacing: 0;-premailer-width: 570px;background-color: #ffffff;border-color: #e8e5ef;border-radius: 2px;border-width: 1px;box-shadow: 0 2px 0 rgba(0, 0, 150, 0.025), 2px 4px 0 rgba(0, 0, 150, 0.015);margin: 0 auto;padding: 0;width: 570px">
<tbody>
<tr>
<td style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;max-width: 100vw;padding: 32px">
<h1 style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;color: #3d4852;font-size: 18px;font-weight: bold;margin-top: 0;text-align: left">Merhaba,</h1>
<p style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;font-size: 16px;line-height: 1.5em;margin-top: 0;text-align: left">Yapmış olduğunuz başvuru onaylanmıştır.</p>
<p style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;font-size: 16px;line-height: 1.5em;margin-top: 0;text-align: left">Sistem üzerinden mağazanızı oluşturmadan önce aşağıdaki butona tıklayarak üyeliğinizi oluşturabilirsiniz.</p>
<br />
<p style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;font-size: 16px;line-height: 1.5em;margin-top: 0;text-align: left">Sevgilerle,<br>
MeetWeb Müşteri Hizmetleri</p>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
<tr>
<td style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative">
<table align="center" width="570" cellpadding="0" cellspacing="0" role="presentation" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;-premailer-cellpadding: 0;-premailer-cellspacing: 0;-premailer-width: 570px;margin: 0 auto;padding: 0;text-align: center;width: 570px">
<tbody>
<tr>
<td align="center" style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;max-width: 100vw;padding: 32px">
<p style="box-sizing: border-box;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';position: relative;line-height: 1.5em;margin-top: 0;color: #b0adc5;font-size: 12px;text-align: center">© 2021 MeetWeb. Tüm hakları saklıdır.</p>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
END;
echo $icerik;