You can use shortcode attributes in the following way:
// The shortcode function
function mailchimp_form_shortcode( $atts = [], $content = null ) {
$attributes = shortcode_atts( array(
'option1' => 'no',
'option2' => 'no',
), $atts );
$mc_form = '
<div id="mc_embed_signup">
<form action="xxxxxxx" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form">
<div id="mc_embed_signup_scroll">
<div class="container">
<div class="mc-field-group">
<label for="mce-EMAIL">E-mail: <span class="asterisk"> (required)</span></label>
<input type="email" value="" name="EMAIL" class="required email" id="mce-EMAIL" placeholder="e-mail">
</div>';
if( $attributes['option1'] === 'yes' ) {
$mc_form .= '
<!-- Subscribe options -->
<p class="subscribe-options"> - Category 1 <!-- Subscribe option 1 -->
<span>
<input type="checkbox" value="1234" name="group[1111][1234]" id="mce-group[1111]-1111-0"> daily<span class="asterisk">*</span>
<input type="checkbox" value="1235" name="group[1111][1235]" id="mce-group[1111]-1111-1"> weekly
</span>
</p>';
}
if( $attributes['option2'] === 'yes' ) {
$mc_form .= '
<p class="subscribe-options"> - Category 2 <!-- Subscribe option 2 -->
<span>
<input type="checkbox" value="1236" name="group[1111][1236]" id="mce-group[1111]-1111-2"> daily<span class="asterisk">*</span>
<input type="checkbox" value="1237" name="group[1111][1237]" id="mce-group[1111]-1111-3"> weekly
</span>
</p>';
}
$mc_form .= '
</div> <!-- .container -->
<div id="mce-responses" class="clear">
<div class="response" id="mce-error-response" style="display:none"></div>
<div class="response" id="mce-success-response" style="display:none"></div>
</div> <!-- real people should not fill this in and expect good things - do not remove this or risk form bot signups-->
<div style="position: absolute; left: -5000px;" aria-hidden="true"><input type="text" name="xxxxxxx" tabindex="-1" value=""></div>
<div class="clear">
<input type="submit" value="Subscribe" name="subscribe" id="mc-embedded-subscribe" class="button">
</div>
</div>
</form>
</div>';
return $mc_form;
}
add_shortcode('mailchimp', 'mailchimp_form_shortcode');
Pay attention at .=
notation.