wp_mail( string|array $to, string $subject, string $message, string|array $headers = '', string|array $attachments = array() )

function wp_mail( $to, $subject, $message, $headers = '', $attachments = array() ) {
	// Compact the input, apply the filters, and extract them back out

	/**
	 * Filters the wp_mail() arguments.
	 *
	 * @since WP-2.2.0
	 *
	 * @param array $args {
	 *     Array of the `wp_mail()` arguments.
	 *
	 *     @type string|string[] $to          Array or comma-separated list of email addresses to send message.
	 *     @type string          $subject     Email subject.
	 *     @type string          $message     Message contents.
	 *     @type string|string[] $headers     Additional headers.
	 *     @type string|string[] $attachments Paths to files to attach.
	 * }
	 */
	$atts = apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) );

	/**
	 * Filters whether to preempt sending an email.
	 *
	 * Returning a non-null value will short-circuit {@see wp_mail()}, returning
	 * that value instead. A boolean return value should be used to indicate whether
	 * the email was successfully sent.
	 *
	 * @since WP-5.7.0
	 *
	 * @param null|bool $return Short-circuit return value.
	 * @param array     $atts {
	 *     Array of the `wp_mail()` arguments.
	 *
	 *     @type string|string[] $to          Array or comma-separated list of email addresses to send message.
	 *     @type string          $subject     Email subject.
	 *     @type string          $message     Message contents.
	 *     @type string|string[] $headers     Additional headers.
	 *     @type string|string[] $attachments Paths to files to attach.
	 * }
	 */
	$pre_wp_mail = apply_filters( 'pre_wp_mail', null, $atts );

	if ( null !== $pre_wp_mail ) {
		return $pre_wp_mail;
	}

	if ( isset( $atts['to'] ) ) {
		$to = $atts['to'];
	}

	if ( !is_array( $to ) ) {
		$to = explode( ',', $to );
	}

	if ( isset( $atts['subject'] ) ) {
		$subject = $atts['subject'];
	}

	if ( isset( $atts['message'] ) ) {
		$message = $atts['message'];
	}

	if ( isset( $atts['headers'] ) ) {
		$headers = $atts['headers'];
	}

	if ( isset( $atts['attachments'] ) ) {
		$attachments = $atts['attachments'];
	}

	if ( ! is_array( $attachments ) ) {
		$attachments = explode( "\n", str_replace( "\r\n", "\n", $attachments ) );
	}
	global $phpmailer;

	// (Re)create it, if it's gone missing
	if ( ! ( $phpmailer instanceof PHPMailer ) ) {
		require_once ABSPATH . WPINC . '/class-phpmailer.php';
		require_once ABSPATH . WPINC . '/class-smtp.php';
		$phpmailer = new PHPMailer( true );
	}

	// Headers
	$cc = $bcc = $reply_to = array();

	if ( empty( $headers ) ) {
		$headers = array();
	} else {
		if ( !is_array( $headers ) ) {
			// Explode the headers out, so this function can take both
			// string headers and an array of headers.
			$tempheaders = explode( "\n", str_replace( "\r\n", "\n", $headers ) );
		} else {
			$tempheaders = $headers;
		}
		$headers = array();

		// If it's actually got contents
		if ( !empty( $tempheaders ) ) {
			// Iterate through the raw headers
			foreach ( (array) $tempheaders as $header ) {
				if ( strpos($header, ':') === false ) {
					if ( false !== stripos( $header, 'boundary=' ) ) {
						$parts = preg_split('/boundary=/i', trim( $header ) );
						$boundary = trim( str_replace( array( "'", '"' ), '', $parts[1] ) );
					}
					continue;
				}
				// Explode them out
				list( $name, $content ) = explode( ':', trim( $header ), 2 );

				// Cleanup crew
				$name    = trim( $name    );
				$content = trim( $content );

				switch ( strtolower( $name ) ) {
					// Mainly for legacy -- process a From: header if it's there
					case 'from':
						$bracket_pos = strpos( $content, '<' );
						if ( $bracket_pos !== false ) {
							// Text before the bracketed email is the "From" name.
							if ( $bracket_pos > 0 ) {
								$from_name = substr( $content, 0, $bracket_pos - 1 );
								$from_name = str_replace( '"', '', $from_name );
								$from_name = trim( $from_name );
							}

							$from_email = substr( $content, $bracket_pos + 1 );
							$from_email = str_replace( '>', '', $from_email );
							$from_email = trim( $from_email );

						// Avoid setting an empty $from_email.
						} elseif ( '' !== trim( $content ) ) {
							$from_email = trim( $content );
						}
						break;
					case 'content-type':
						if ( strpos( $content, ';' ) !== false ) {
							list( $type, $charset_content ) = explode( ';', $content );
							$content_type = trim( $type );
							if ( false !== stripos( $charset_content, 'charset=' ) ) {
								$charset = trim( str_replace( array( 'charset=', '"' ), '', $charset_content ) );
							} elseif ( false !== stripos( $charset_content, 'boundary=' ) ) {
								$boundary = trim( str_replace( array( 'BOUNDARY=', 'boundary=', '"' ), '', $charset_content ) );
								$charset = '';
							}

						// Avoid setting an empty $content_type.
						} elseif ( '' !== trim( $content ) ) {
							$content_type = trim( $content );
						}
						break;
					case 'cc':
						$cc = array_merge( (array) $cc, explode( ',', $content ) );
						break;
					case 'bcc':
						$bcc = array_merge( (array) $bcc, explode( ',', $content ) );
						break;
					case 'reply-to':
						$reply_to = array_merge( (array) $reply_to, explode( ',', $content ) );
						break;
					default:
						// Add it to our grand headers array
						$headers[trim( $name )] = trim( $content );
						break;
				}
			}
		}
	}

	// Empty out the values that may be set
	$phpmailer->clearAllRecipients();
	$phpmailer->clearAttachments();
	$phpmailer->clearCustomHeaders();
	$phpmailer->clearReplyTos();

	// From email and name
	// If we don't have a name from the input headers
	if ( !isset( $from_name ) )
		$from_name = 'ClassicPress';

	/* If we don't have an email from the input headers default to classicpress@$sitename
	 * Some hosts will block outgoing mail from this address if it doesn't exist but
	 * there's no easy alternative. Defaulting to admin_email might appear to be another
	 * option but some hosts may refuse to relay mail from an unknown domain. See
	 * https://core.trac.wordpress.org/ticket/5007.
	 */

	if ( !isset( $from_email ) ) {
		// Get the site domain and get rid of www.
		$sitename = strtolower( $_SERVER['SERVER_NAME'] );
		if ( substr( $sitename, 0, 4 ) == 'www.' ) {
			$sitename = substr( $sitename, 4 );
		}

		$from_email = 'classicpress@' . $sitename;
	}

	/**
	 * Filters the email address to send from.
	 *
	 * @since WP-2.2.0
	 *
	 * @param string $from_email Email address to send from.
	 */
	$from_email = apply_filters( 'wp_mail_from', $from_email );

	/**
	 * Filters the name to associate with the "from" email address.
	 *
	 * @since WP-2.3.0
	 *
	 * @param string $from_name Name associated with the "from" email address.
	 */
	$from_name = apply_filters( 'wp_mail_from_name', $from_name );

	try {
		$phpmailer->setFrom( $from_email, $from_name, false );
	} catch ( phpmailerException $e ) {
		$mail_error_data = compact( 'to', 'subject', 'message', 'headers', 'attachments' );
		$mail_error_data['phpmailer_exception_code'] = $e->getCode();

		/** This filter is documented in wp-includes/pluggable.php */
		do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );

		return false;
	}

	// Set mail's subject and body
	$phpmailer->Subject = $subject;
	$phpmailer->Body    = $message;

	// Set destination addresses, using appropriate methods for handling addresses
	$address_headers = compact( 'to', 'cc', 'bcc', 'reply_to' );

	foreach ( $address_headers as $address_header => $addresses ) {
		if ( empty( $addresses ) ) {
			continue;
		}

		foreach ( (array) $addresses as $address ) {
			try {
				// Break $recipient into name and address parts if in the format "Foo <[email protected]>"
				$recipient_name = '';

				if ( preg_match( '/(.*)<(.+)>/', $address, $matches ) ) {
					if ( count( $matches ) == 3 ) {
						$recipient_name = $matches[1];
						$address        = $matches[2];
					}
				}

				switch ( $address_header ) {
					case 'to':
						$phpmailer->addAddress( $address, $recipient_name );
						break;
					case 'cc':
						$phpmailer->addCc( $address, $recipient_name );
						break;
					case 'bcc':
						$phpmailer->addBcc( $address, $recipient_name );
						break;
					case 'reply_to':
						$phpmailer->addReplyTo( $address, $recipient_name );
						break;
				}
			} catch ( phpmailerException $e ) {
				continue;
			}
		}
	}

	// Set to use PHP's mail()
	$phpmailer->isMail();

	// Set Content-Type and charset
	// If we don't have a content-type from the input headers
	if ( !isset( $content_type ) )
		$content_type = 'text/plain';

	/**
	 * Filters the wp_mail() content type.
	 *
	 * @since WP-2.3.0
	 *
	 * @param string $content_type Default wp_mail() content type.
	 */
	$content_type = apply_filters( 'wp_mail_content_type', $content_type );

	$phpmailer->ContentType = $content_type;

	// Set whether it's plaintext, depending on $content_type
	if ( 'text/html' == $content_type )
		$phpmailer->isHTML( true );

	// If we don't have a charset from the input headers
	if ( !isset( $charset ) )
		$charset = get_bloginfo( 'charset' );

	// Set the content-type and charset

	/**
	 * Filters the default wp_mail() charset.
	 *
	 * @since WP-2.3.0
	 *
	 * @param string $charset Default email charset.
	 */
	$phpmailer->CharSet = apply_filters( 'wp_mail_charset', $charset );

	// Set custom headers.
	if ( ! empty( $headers ) ) {
		foreach ( (array) $headers as $name => $content ) {
			// Only add custom headers not added automatically by PHPMailer.
			if ( ! in_array( $name, array( 'MIME-Version', 'X-Mailer' ) ) ) {
				$phpmailer->addCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
			}
		}

		if ( false !== stripos( $content_type, 'multipart' ) && ! empty($boundary) )
			$phpmailer->addCustomHeader( sprintf( "Content-Type: %s;\n\t boundary=\"%s\"", $content_type, $boundary ) );
	}

	if ( !empty( $attachments ) ) {
		foreach ( $attachments as $attachment ) {
			try {
				$phpmailer->addAttachment($attachment);
			} catch ( phpmailerException $e ) {
				continue;
			}
		}
	}

	/**
	 * Fires after PHPMailer is initialized.
	 *
	 * @since WP-2.2.0
	 *
	 * @param PHPMailer $phpmailer The PHPMailer instance (passed by reference).
	 */
	do_action_ref_array( 'phpmailer_init', array( &$phpmailer ) );

	// Send!
	try {
		return $phpmailer->send();
	} catch ( phpmailerException $e ) {

		$mail_error_data = compact( 'to', 'subject', 'message', 'headers', 'attachments' );
		$mail_error_data['phpmailer_exception_code'] = $e->getCode();

		/**
		 * Fires after a phpmailerException is caught.
		 *
		 * @since WP-4.4.0
		 *
		 * @param WP_Error $error A WP_Error object with the phpmailerException message, and an array
		 *                        containing the mail recipient, subject, message, headers, and attachments.
		 */
		do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $e->getMessage(), $mail_error_data ) );

		return false;
	}
}