Check if a site is a WordPress site. Does only basic checks and doesn't check the site endpoints.

is_wp_site.php

<?php

$list = [
	'https://ma.tt',
	'https://google.com',
	'https://wordpress.org'
];


function is_wp_site( $site ) {
	$content = file_get_contents( $site );

	libxml_use_internal_errors( true );

	$dom = new DOMDocument;
	$dom->loadHTML( $content );
	$x_path = new DOMXPath( $dom );

	// check links (exposing API)
	$query = $x_path->query( '//link[@rel="https://api.w.org/"]' );
	if ( $query->length ) {
		return true;
	}

	// check head scripts
	$query = $x_path->query( '//head//script' );
	foreach ( $query as $element ) {
		if ( url_matches_wp_url( $element->getAttribute( 'src' ) ) ) {
			return true;
		}
	}

	// check all scripts
	$query = $x_path->query( '//script' );
	foreach ( $query as $element ) {
		if ( url_matches_wp_url( $element->getAttribute( 'src' ) ) ) {
			return true;
		}
	}

	// check all style
	$query = $x_path->query( '//link[@rel="stylesheet"]' );
	foreach ( $query as $element ) {
		if ( url_matches_wp_url( $element->getAttribute( 'href' ) ) ) {
			return true;
		}
	}

	if ( fulltext_matches_wp( $content ) ) {
		return true;
	}

	return false;
}

function url_matches_wp_url( $url ) {
	$matches = [
		'\/blog-content\/',
		'\/wp-content\/',
		'\/wp-includes\/',
		'\/js\/devicepx-jetpack\.js' // included Jetpack
	];

	foreach ( $matches as $check ) {
		if ( preg_match( '!' . $check . '!uis', $url ) ) {
			return true;
		}
	}

	return false;
}

function fulltext_matches_wp( $content ) {
	// check for WP Emoji settings
	if ( preg_match( '!window\._wpemojiSettings!uis', $content ) ) {
		return true;
	}

	// check for smileys CSS class
	if ( preg_match( '!img\.wp-smiley!uis', $content ) ) {
		return true;
	}

	return false;
}

foreach ( $list as $site ) {
	var_dump( 'Is ' . $site . ' a WordPress site?: ' . ( is_wp_site( $site ) ? 'YES' : 'NO' ) );
}