Replace mustache placeholders in Child Theme starter content

child-functions.php

<?php

/**
 * Filter starter content placeholders for new sites using this Child Theme.
 *
 * @var array
 */
add_filter( 'mysite_starter_content_placeholders', function () {

	return [
		'city'  => 'Chicago',
		'phone' => '(999) 999-9999',
	];

} );

parent-functions.php

<?php

$content = [
	'posts' => [
		'about' => [
			'post_type'    => 'page',
			'post_title'   => 'About {{city}}',
			'post_content' => '<p>To learn more, call {{phone}}.</p>',
		],
	],
];

$defaults = [
	'city'  => 'New York',
	'phone' => '(555) 555-5555',
];

/**
 * Starter content for new sites using this theme.
 *
 * @link https://make.wordpress.org/core/2016/11/30/starter-content-for-themes-in-4-7/
 */
add_theme_support( 'starter-content', function () use ( $content, $defaults ) {

	$content = json_encode( $content ); // Array to JSON string

	$placeholders = (array) apply_filters( 'mysite_starter_content_placeholders', [] );
	$placeholders = wp_parse_args( $placeholders, (array) $defaults );

	foreach ( $placeholders as $placeholder => $value ) {

		$content = str_replace( sprintf( '{{%s}}', $placeholder ), $value, $content );

	}

	return json_decode( $content, true ); // JSON string to assoc array

} );