Composer: Declare PHP Extension dependencies

[swissspidy]

While looking at #1834 I noticed that we currently don't declare PHP extension and library requirements in composer.json.

Off the top of my head, we'd need the following platform dependencies:

{
  "require": {
    "ext-intl": "*",
    "ext-libxml": "*",
    "ext-dom": "*"
  }
}

Perhaps even "lib-icu": "*", since that's being used by ext-intl.

[westonruter]

Excellent. I didn't know those could be formalized as such.

Also required is iconv.

Relates to this code:

amp-wp/amp.php

Lines 41 to 89 in 401a4e9

	/**
	* Print admin notice regarding DOM extension is not installed.
	*
	* @since 1.0
	*/
	function _amp_print_php_dom_document_notice() {
	?>
	<div class="notice notice-error">
	<p>
	<?php
	printf(
	/* translators: %s: PHP extension name */
	esc_html__( 'The AMP plugin requires the %s extension in PHP. Please contact your host to install this extension.', 'amp' ),
	'DOM'
	);
	?>
	</p>
	</div>
	<?php
	}
	if ( ! class_exists( 'DOMDocument' ) ) {
	add_action( 'admin_notices', '_amp_print_php_dom_document_notice' );
	return;
	}
	/**
	* Print admin notice regarding DOM extension is not installed.
	*
	* @since 1.0.1
	*/
	function _amp_print_php_missing_iconv_notice() {
	?>
	<div class="notice notice-error">
	<p>
	<?php
	printf(
	/* translators: %s: PHP extension name */
	esc_html__( 'The AMP plugin requires the %s extension in PHP. Please contact your host to install this extension.', 'amp' ),
	'iconv'
	);
	?>
	</p>
	</div>
	<?php
	}
	if ( ! function_exists( 'iconv' ) ) {
	add_action( 'admin_notices', '_amp_print_php_missing_iconv_notice' );
	return;
	}