diff --git a/wp-includes/class-wp-theme-json.php b/wp-includes/class-wp-theme-json.php
index df5bd20d84..d93b4eea67 100644
--- a/wp-includes/class-wp-theme-json.php
+++ b/wp-includes/class-wp-theme-json.php
@@ -461,6 +461,8 @@ class WP_Theme_JSON {
 	public static function get_element_class_name( $element ) {
 		$class_name = '';
 
+		// TODO: Replace array_key_exists() with isset() check once WordPress drops
+		// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 		if ( array_key_exists( $element, static::__EXPERIMENTAL_ELEMENT_CLASS_NAMES ) ) {
 			$class_name = static::__EXPERIMENTAL_ELEMENT_CLASS_NAMES[ $element ];
 		}
@@ -519,7 +521,10 @@ class WP_Theme_JSON {
 		$nodes = static::get_setting_nodes( $this->theme_json );
 		foreach ( $nodes as $node ) {
 			foreach ( static::PRESETS_METADATA as $preset_metadata ) {
-				$path   = array_merge( $node['path'], $preset_metadata['path'] );
+				$path = $node['path'];
+				foreach ( $preset_metadata['path'] as $subpath ) {
+					$path[] = $subpath;
+				}
 				$preset = _wp_array_get( $this->theme_json, $path, null );
 				if ( null !== $preset ) {
 					// If the preset is not already keyed by origin.
@@ -608,6 +613,7 @@ class WP_Theme_JSON {
 		 */
 		$styles_non_top_level = static::VALID_STYLES;
 		foreach ( array_keys( $styles_non_top_level ) as $section ) {
+			// array_key_exists() needs to be used instead of isset() because the value can be null.
 			if ( array_key_exists( $section, $styles_non_top_level ) && is_array( $styles_non_top_level[ $section ] ) ) {
 				foreach ( array_keys( $styles_non_top_level[ $section ] ) as $prop ) {
 					if ( 'top' === $styles_non_top_level[ $section ][ $prop ] ) {
@@ -631,6 +637,8 @@ class WP_Theme_JSON {
 		foreach ( $valid_element_names as $element ) {
 			$schema_styles_elements[ $element ] = $styles_non_top_level;
 
+			// TODO: Replace array_key_exists() with isset() check once WordPress drops
+			// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 			if ( array_key_exists( $element, static::VALID_ELEMENT_PSEUDO_SELECTORS ) ) {
 				foreach ( static::VALID_ELEMENT_PSEUDO_SELECTORS[ $element ] as $pseudo_selector ) {
 					$schema_styles_elements[ $element ][ $pseudo_selector ] = $styles_non_top_level;
@@ -1273,8 +1281,12 @@ class WP_Theme_JSON {
 
 			$selector = $metadata['selector'];
 
-			$node         = _wp_array_get( $this->theme_json, $metadata['path'], array() );
-			$declarations = array_merge( static::compute_preset_vars( $node, $origins ), static::compute_theme_vars( $node ) );
+			$node                    = _wp_array_get( $this->theme_json, $metadata['path'], array() );
+			$declarations            = static::compute_preset_vars( $node, $origins );
+			$theme_vars_declarations = static::compute_theme_vars( $node );
+			foreach ( $theme_vars_declarations as $theme_vars_declaration ) {
+				$declarations[] = $theme_vars_declaration;
+			}
 
 			$stylesheet .= static::to_ruleset( $selector, $declarations );
 		}
@@ -1601,11 +1613,11 @@ class WP_Theme_JSON {
 			);
 
 			if ( is_array( $value ) ) {
-				$new_prefix = $new_key . $token;
-				$result     = array_merge(
-					$result,
-					static::flatten_tree( $value, $new_prefix, $token )
-				);
+				$new_prefix        = $new_key . $token;
+				$flattened_subtree = static::flatten_tree( $value, $new_prefix, $token );
+				foreach ( $flattened_subtree as $subtree_key => $subtree_value ) {
+					$result[ $subtree_key ] = $subtree_value;
+				}
 			} else {
 				$result[ $new_key ] = $value;
 			}
@@ -1667,6 +1679,8 @@ class WP_Theme_JSON {
 			if ( is_array( $value_path ) ) {
 				$path_string = implode( '.', $value_path );
 				if (
+					// TODO: Replace array_key_exists() with isset() check once WordPress drops
+					// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 					array_key_exists( $path_string, static::PROTECTED_PROPERTIES ) &&
 					_wp_array_get( $settings, static::PROTECTED_PROPERTIES[ $path_string ], null ) === null
 				) {
@@ -1742,7 +1756,7 @@ class WP_Theme_JSON {
 		 * where the values is an array with a "ref" key, pointing to a path.
 		 * For example: { "ref": "style.color.background" } => "#fff".
 		 */
-		if ( is_array( $value ) && array_key_exists( 'ref', $value ) ) {
+		if ( is_array( $value ) && isset( $value['ref'] ) ) {
 			$value_path = explode( '.', $value['ref'] );
 			$ref_value  = _wp_array_get( $theme_json, $value_path );
 			// Only use the ref value if we find anything.
@@ -1750,7 +1764,7 @@ class WP_Theme_JSON {
 				$value = $ref_value;
 			}
 
-			if ( is_array( $ref_value ) && array_key_exists( 'ref', $ref_value ) ) {
+			if ( is_array( $ref_value ) && isset( $ref_value['ref'] ) ) {
 				$path_string      = json_encode( $path );
 				$ref_value_string = json_encode( $ref_value );
 				_doing_it_wrong(
@@ -1886,6 +1900,8 @@ class WP_Theme_JSON {
 				);
 
 				// Handle any pseudo selectors for the element.
+				// TODO: Replace array_key_exists() with isset() check once WordPress drops
+				// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 				if ( array_key_exists( $element, static::VALID_ELEMENT_PSEUDO_SELECTORS ) ) {
 					foreach ( static::VALID_ELEMENT_PSEUDO_SELECTORS[ $element ] as $pseudo_selector ) {
 
@@ -1905,7 +1921,10 @@ class WP_Theme_JSON {
 			return $nodes;
 		}
 
-		$nodes = array_merge( $nodes, static::get_block_nodes( $theme_json ) );
+		$block_nodes = static::get_block_nodes( $theme_json );
+		foreach ( $block_nodes as $block_node ) {
+			$nodes[] = $block_node;
+		}
 
 		/**
 		 * Filters the list of style nodes with metadata.
@@ -1982,6 +2001,8 @@ class WP_Theme_JSON {
 					);
 
 					// Handle any pseudo selectors for the element.
+					// TODO: Replace array_key_exists() with isset() check once WordPress drops
+					// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 					if ( array_key_exists( $element, static::VALID_ELEMENT_PSEUDO_SELECTORS ) ) {
 						foreach ( static::VALID_ELEMENT_PSEUDO_SELECTORS[ $element ] as $pseudo_selector ) {
 							if ( isset( $theme_json['styles']['blocks'][ $name ]['elements'][ $element ][ $pseudo_selector ] ) ) {
@@ -2035,7 +2056,9 @@ class WP_Theme_JSON {
 					// the feature selector. This may occur when multiple block
 					// support features use the same custom selector.
 					if ( isset( $feature_declarations[ $feature_selector ] ) ) {
-						$feature_declarations[ $feature_selector ] = array_merge( $feature_declarations[ $feature_selector ], $new_feature_declarations );
+						foreach ( $new_feature_declarations as $new_feature_declaration ) {
+							$feature_declarations[ $feature_selector ][] = $feature_declaration;
+						}
 					} else {
 						$feature_declarations[ $feature_selector ] = $new_feature_declarations;
 					}
@@ -2059,6 +2082,8 @@ class WP_Theme_JSON {
 
 		$element_pseudo_allowed = array();
 
+		// TODO: Replace array_key_exists() with isset() check once WordPress drops
+		// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 		if ( array_key_exists( $current_element, static::VALID_ELEMENT_PSEUDO_SELECTORS ) ) {
 			$element_pseudo_allowed = static::VALID_ELEMENT_PSEUDO_SELECTORS[ $current_element ];
 		}
@@ -2084,6 +2109,8 @@ class WP_Theme_JSON {
 		 * Otherwise just compute the styles for the default selector as normal.
 		 */
 		if ( $pseudo_selector && isset( $node[ $pseudo_selector ] ) &&
+			// TODO: Replace array_key_exists() with isset() check once WordPress drops
+			// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 			array_key_exists( $current_element, static::VALID_ELEMENT_PSEUDO_SELECTORS )
 			&& in_array( $pseudo_selector, static::VALID_ELEMENT_PSEUDO_SELECTORS[ $current_element ], true )
 		) {
@@ -2283,11 +2310,11 @@ class WP_Theme_JSON {
 		$nodes        = static::get_setting_nodes( $incoming_data );
 		$slugs_global = static::get_default_slugs( $this->theme_json, array( 'settings' ) );
 		foreach ( $nodes as $node ) {
-			$slugs_node = static::get_default_slugs( $this->theme_json, $node['path'] );
-			$slugs      = array_merge_recursive( $slugs_global, $slugs_node );
-
 			// Replace the spacing.units.
-			$path    = array_merge( $node['path'], array( 'spacing', 'units' ) );
+			$path   = $node['path'];
+			$path[] = 'spacing';
+			$path[] = 'units';
+
 			$content = _wp_array_get( $incoming_data, $path, null );
 			if ( isset( $content ) ) {
 				_wp_array_set( $this->theme_json, $path, $content );
@@ -2298,19 +2325,25 @@ class WP_Theme_JSON {
 				$override_preset = ! static::get_metadata_boolean( $this->theme_json['settings'], $preset['prevent_override'], true );
 
 				foreach ( static::VALID_ORIGINS as $origin ) {
-					$base_path = array_merge( $node['path'], $preset['path'] );
-					$path      = array_merge( $base_path, array( $origin ) );
-					$content   = _wp_array_get( $incoming_data, $path, null );
+					$base_path = $node['path'];
+					foreach ( $preset['path'] as $leaf ) {
+						$base_path[] = $leaf;
+					}
+
+					$path   = $base_path;
+					$path[] = $origin;
+
+					$content = _wp_array_get( $incoming_data, $path, null );
 					if ( ! isset( $content ) ) {
 						continue;
 					}
 
 					if ( 'theme' === $origin && $preset['use_default_names'] ) {
-						foreach ( $content as &$item ) {
-							if ( ! array_key_exists( 'name', $item ) ) {
+						foreach ( $content as $key => $item ) {
+							if ( ! isset( $item['name'] ) ) {
 								$name = static::get_name_from_defaults( $item['slug'], $base_path );
 								if ( null !== $name ) {
-									$item['name'] = $name;
+									$content[ $key ]['name'] = $name;
 								}
 							}
 						}
@@ -2322,6 +2355,9 @@ class WP_Theme_JSON {
 					) {
 						_wp_array_set( $this->theme_json, $path, $content );
 					} else {
+						$slugs_node = static::get_default_slugs( $this->theme_json, $node['path'] );
+						$slugs      = array_merge_recursive( $slugs_global, $slugs_node );
+
 						$slugs_for_preset = _wp_array_get( $slugs, $preset['path'], array() );
 						$content          = static::filter_slugs( $content, $slugs_for_preset );
 						_wp_array_set( $this->theme_json, $path, $content );
@@ -2434,7 +2470,12 @@ class WP_Theme_JSON {
 		$slugs = array();
 
 		foreach ( static::PRESETS_METADATA as $metadata ) {
-			$path   = array_merge( $node_path, $metadata['path'], array( 'default' ) );
+			$path = $node_path;
+			foreach ( $metadata['path'] as $leaf ) {
+				$path[] = $leaf;
+			}
+			$path[] = 'default';
+
 			$preset = _wp_array_get( $data, $path, null );
 			if ( ! isset( $preset ) ) {
 				continue;
@@ -2463,7 +2504,8 @@ class WP_Theme_JSON {
 	 * @return string|null
 	 */
 	protected function get_name_from_defaults( $slug, $base_path ) {
-		$path            = array_merge( $base_path, array( 'default' ) );
+		$path            = $base_path;
+		$path[]          = 'default';
 		$default_content = _wp_array_get( $this->theme_json, $path, null );
 		if ( ! $default_content ) {
 			return null;
@@ -2539,6 +2581,8 @@ class WP_Theme_JSON {
 			 * $output is stripped of pseudo selectors. Re-add and process them
 			 * or insecure styles here.
 			 */
+			// TODO: Replace array_key_exists() with isset() check once WordPress drops
+			// support for PHP 5.6. See https://core.trac.wordpress.org/ticket/57067.
 			if ( array_key_exists( $current_element, static::VALID_ELEMENT_PSEUDO_SELECTORS ) ) {
 				foreach ( static::VALID_ELEMENT_PSEUDO_SELECTORS[ $current_element ] as $pseudo_selector ) {
 					if ( isset( $input[ $pseudo_selector ] ) ) {
@@ -2593,8 +2637,9 @@ class WP_Theme_JSON {
 		$output = array();
 		foreach ( static::PRESETS_METADATA as $preset_metadata ) {
 			foreach ( static::VALID_ORIGINS as $origin ) {
-				$path_with_origin = array_merge( $preset_metadata['path'], array( $origin ) );
-				$presets          = _wp_array_get( $input, $path_with_origin, null );
+				$path_with_origin   = $preset_metadata['path'];
+				$path_with_origin[] = $origin;
+				$presets            = _wp_array_get( $input, $path_with_origin, null );
 				if ( null === $presets ) {
 					continue;
 				}
@@ -2839,7 +2884,10 @@ class WP_Theme_JSON {
 		 */
 		foreach ( $nodes as $node ) {
 			foreach ( static::PRESETS_METADATA as $preset_metadata ) {
-				$path   = array_merge( $node['path'], $preset_metadata['path'] );
+				$path = $node['path'];
+				foreach ( $preset_metadata['path'] as $preset_metadata_path ) {
+					$path[] = $preset_metadata_path;
+				}
 				$preset = _wp_array_get( $output, $path, null );
 				if ( null === $preset ) {
 					continue;
@@ -2873,7 +2921,10 @@ class WP_Theme_JSON {
 		foreach ( $nodes as $node ) {
 			$all_opt_ins_are_set = true;
 			foreach ( static::APPEARANCE_TOOLS_OPT_INS as $opt_in_path ) {
-				$full_path = array_merge( $node['path'], $opt_in_path );
+				$full_path = $node['path'];
+				foreach ( $opt_in_path as $opt_in_path_item ) {
+					$full_path[] = $opt_in_path_item;
+				}
 				// Use "unset prop" as a marker instead of "null" because
 				// "null" can be a valid value for some props (e.g. blockGap).
 				$opt_in_value = _wp_array_get( $output, $full_path, 'unset prop' );
@@ -2884,9 +2935,14 @@ class WP_Theme_JSON {
 			}
 
 			if ( $all_opt_ins_are_set ) {
-				_wp_array_set( $output, array_merge( $node['path'], array( 'appearanceTools' ) ), true );
+				$node_path_with_appearance_tools   = $node['path'];
+				$node_path_with_appearance_tools[] = 'appearanceTools';
+				_wp_array_set( $output, $node_path_with_appearance_tools, true );
 				foreach ( static::APPEARANCE_TOOLS_OPT_INS as $opt_in_path ) {
-					$full_path = array_merge( $node['path'], $opt_in_path );
+					$full_path = $node['path'];
+					foreach ( $opt_in_path as $opt_in_path_item ) {
+						$full_path[] = $opt_in_path_item;
+					}
 					// Use "unset prop" as a marker instead of "null" because
 					// "null" can be a valid value for some props (e.g. blockGap).
 					$opt_in_value = _wp_array_get( $output, $full_path, 'unset prop' );
@@ -3037,7 +3093,10 @@ class WP_Theme_JSON {
 			$slug += 10;
 		}
 
-		$spacing_sizes = array_merge( $below_sizes, $above_sizes );
+		$spacing_sizes = $below_sizes;
+		foreach ( $above_sizes as $above_sizes_item ) {
+			$spacing_sizes[] = $above_sizes_item;
+		}
 
 		// If there are 7 or less steps in the scale revert to numbers for labels instead of t-shirt sizes.
 		if ( $spacing_scale['steps'] <= 7 ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 8567d156df..2350c21884 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-54803';
+$wp_version = '6.2-alpha-54804';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.