@param  array  $array       The array.
	 * @param  bool   $outerQuotes Whether outer quotes should be added.
	 * @return string              The imploded array.
	 */
	public function implodeWhereIn( $array, $outerQuotes = false ) {
		// Reset the keys first in case there is no 0 index.
		$array = array_values( $array );

		if ( ! isset( $array[0] ) ) {
			return '';
		}

		if ( is_numeric( $array[0] ) ) {
			return implode( ', ', $array );
		}

		return $outerQuotes ? "'" . implode( "', '", $array ) . "'" : implode( "', '", $array );
	}

	/**
	 * Returns an imploded string of placeholders for usage in a WPDB prepare statement.
	 *
	 * @since 4.1.9
	 *
	 * @param  array  $array       The array.
	 * @param  string $placeholder The placeholder (e.g. "%s" or "%d").
	 * @return string              The imploded string with placeholders.
	 */
	public function implodePlaceholders( $array, $placeholder = '%s' ) {
		return implode( ', ', array_fill( 0, count( $array ), $placeholder ) );
	}

	/**
	 * Verifies that a string is indeed a valid regular expression.
	 *
	 * @since 4.2.1
	 *
	 * @return boolean True if the string is a valid regular expression.
	 */
	public function isValidRegex( $pattern ) {
		// Set a custom error handler to prevent throwing errors on a bad Regular Expression.
		set_error_handler( function() {}, E_WARNING );

		$isValid = true;

		if ( false === preg_match( $pattern, '' ) ) {
			$isValid = false;
		}

		// Restore the error handler.
		restore_error_handler();

		return $isValid;
	}

	/**
	 * Removes the leading slash(es) from a string.
	 *
	 * @since 4.2.3
	 *
	 * @param  string $string The string.
	 * @return string         The modified string.
	 */
	public function unleadingSlashIt( $string ) {
		return ltrim( $string, '/' );
	}

	/**
	 * Convert the case of the given string.
	 *
	 * @since 4.2.4
	 *
	 * @param  string $string The string.
	 * @param  string $type   The casing ("lower", "title", "sentence").
	 * @return string         The converted string.
	 */
	public function convertCase( $string, $type ) {
		switch ( $type ) {
			case 'lower':
				return strtolower( $string );
			case 'title':
				return $this->toTitleCase( $string );
			case 'sentence':
				return $this->toSentenceCase( $string );
			default:
				return $string;
		}
	}

	/**
	 * Converts the given string to title case.
	 *
	 * @since 4.2.4
	 *
	 * @param  string $string The string.
	 * @return string         The converted string.
	 */
	public function toTitleCase( $string ) {
		// List of common English words that aren't typically modified.
		$exceptions = apply_filters( 'aioseo_title_case_exceptions', [
			'of',
			'a',
			'the',
			'and',
			'an',
			'or',
			'nor',
			'but',
			'is',
			'if',
			'then',
			'else',
			'when',
			'at',
			'from',
			'by',
			'on',
			'off',
			'for',
			'in',
			'out',
			'over',
			'to',
			'into',
			'with'
		] );

		$words = explode( ' ', strtolower( $string ) );

		foreach ( $words as $k => $word ) {
			if ( ! in_array( $word, $exceptions, true ) ) {
				$words[ $k ] = ucfirst( $word );
			}
		}

		$string = implode( ' ', $words );

		return $string;
	}

	/**
	 * Converts the given string to sentence case.
	 *
	 * @since 4.2.4
	 *
	 * @param  string $string The string.
	 * @return string         The converted string.
	 */
	public function toSentenceCase( $string ) {
		$phrases = preg_split( '/([.?!]+)/', $string, -1, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE );

		$convertedString = '';
		foreach ( $phrases as $index => $sentence ) {
			$convertedString .= ( $index & 1 ) === 0 ? ucfirst( strtolower( trim( $sentence ) ) ) : $sentence . ' ';
		}

		return trim( $convertedString );
	}

	/**
	 * Returns the substring with a given start index and length.
	 *
	 * @since 4.2.5
	 *
	 * @param  string $string     The string.
	 * @param  int    $startIndex The start index.
	 * @param  int    $length     The length.
	 * @return string             The substring.
	 */
	public function substring( $string, $startIndex, $length ) {
		return function_exists( 'mb_substr' ) ? mb_substr( $string, $startIndex, $length, $this->getCharset() ) : substr( $string, $startIndex, $length );
	}
}