General: Introduce polyfills for `array_key_first()` and `array_key_last()` added in PHP 7.3.0.

PHP 7.3.0 introduced two new functions: `array_key_first()` and `array_key_last()`. These two functions return the first and last key of each array respectively, without affecting the internal state of the array.

The polyfills make these two functions available for use in Core on PHP versions less than 7.3.0.

Ref:
* PHP RFC https://wiki.php.net/rfc/array_key_first_last
* PHP manual `array_key_first()` https://www.php.net/manual/en/function.array-key-first.php
* PHP manual `array_key_last()` https://www.php.net/manual/en/function.array-key-last.php

Props desrosj, pbearne, costdev, hellofromTonya, ayeshrajans, manzoorwanijk, audrasjb, sergeybiryukov.
Fixes #45055.
Built from https://develop.svn.wordpress.org/trunk@52038


git-svn-id: http://core.svn.wordpress.org/trunk@51630 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/compat.php

@@ -375,6 +375,48 @@ if ( ! function_exists( 'is_iterable' ) ) {
 	}
 }
 
+if ( ! function_exists( 'array_key_first' ) ) {
+	/**
+	 * Polyfill for array_key_first() function added in PHP 7.3.
+	 *
+	 * Get the first key of the given array without affecting
+	 * the internal array pointer.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param array $arr An array.
+	 * @return string|int|null The first key of array if the array
+	 *                         is not empty; `null` otherwise.
+	 */
+	function array_key_first( array $arr ) {
+		foreach ( $arr as $key => $value ) {
+			return $key;
+		}
+	}
+}
+
+if ( ! function_exists( 'array_key_last' ) ) {
+	/**
+	 * Polyfill for `array_key_last()` function added in PHP 7.3.
+	 *
+	 * Get the last key of the given array without affecting the
+	 * internal array pointer.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param array $arr An array.
+	 * @return string|int|null The last key of array if the array
+	 *.                        is not empty; `null` otherwise.
+	 */
+	function array_key_last( array $arr ) {
+		if ( empty( $arr ) ) {
+			return null;
+		}
+		end( $arr );
+		return key( $arr );
+	}
+}
+
 // IMAGETYPE_WEBP constant is only defined in PHP 7.1 or later.
 if ( ! defined( 'IMAGETYPE_WEBP' ) ) {
 	define( 'IMAGETYPE_WEBP', 18 );

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52037';
+$wp_version = '5.9-alpha-52038';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.