I would try using htmlspecialchars().
On the other hand if I was doing this I would probably built and array in PHP and then json_encode that array for output allowing PHP to deal with special characters.
<?php
if ($faq) {
$ld = array(
'@context' => 'https://schema.org',
'@type' => 'FAQPage',
'mainEntity' => array()
);
if (have_rows('faq')) {
while (have_rows('faq')) {
the_row();
$ld['mainEntity'][] = array(
'@type' => 'Question',
'name' => get_sub_field('question'),
'acceptedAnswer' => array(
'@type' => 'Answer',
'text' => get_sub_field('answer')
)
);
}
}
?>
<script type="application/ld+json"><?php echo json_encode($ld); ?></script>
<?php
}
?>
