Source

File: wp-includes/class-wp-date-query.php

public function sanitize_query( $queries, $parent_query = null ) {
		$cleaned_query = array();

		$defaults = array(
			'column'   => 'post_date',
			'compare'  => '=',
			'relation' => 'AND',
		);

		// Numeric keys should always have array values.
		foreach ( $queries as $qkey => $qvalue ) {
			if ( is_numeric( $qkey ) && ! is_array( $qvalue ) ) {
				unset( $queries[ $qkey ] );
			}
		}

		// Each query should have a value for each default key. Inherit from the parent when possible.
		foreach ( $defaults as $dkey => $dvalue ) {
			if ( isset( $queries[ $dkey ] ) ) {
				continue;
			}

			if ( isset( $parent_query[ $dkey ] ) ) {
				$queries[ $dkey ] = $parent_query[ $dkey ];
			} else {
				$queries[ $dkey ] = $dvalue;
			}
		}

		// Validate the dates passed in the query.
		if ( $this->is_first_order_clause( $queries ) ) {
			$this->validate_date_values( $queries );
		}

		foreach ( $queries as $key => $q ) {
			if ( ! is_array( $q ) || in_array( $key, $this->time_keys, true ) ) {
				// This is a first-order query. Trust the values and sanitize when building SQL.
				$cleaned_query[ $key ] = $q;
			} else {
				// Any array without a time key is another query, so we recurse.
				$cleaned_query[] = $this->sanitize_query( $q, $queries );
			}
		}

		return $cleaned_query;
	}