system/Database/BaseBuilder.php
protected function isLiteral(string $str): bool
	{
		$str = trim($str);

		if (empty($str) || ctype_digit($str) || (string) (float) $str === $str ||
				in_array(strtoupper($str), ['TRUE', 'FALSE'], true)
		)
		{
			return true;
		}

		static $_str;

		if (empty($_str))
		{
			$_str = ($this->db->escapeChar !== '"') ? ['"', "'"] : ["'"];
		}

		return in_array($str[0], $_str, true);
	}
protected function isLiteral(string $str): bool
	{
		$str = trim($str);

		if (empty($str) || ctype_digit($str) || (string) (float) $str === $str ||
				in_array(strtoupper($str), ['TRUE', 'FALSE'], true)
		)
		{
			return true;
		}

		static $_str;

		if (empty($_str))
		{
			$_str = ($this->db->escapeChar !== '"') ? ['"', "'"] : ["'"];
		}

		return in_array($str[0], $_str, true);
	}
Is literal

Determines if a string represents a literal value or a field name

@param string $str

@return boolean