Yes, you can definitely apply for a PhD program after a year and a half out of your Master's degree. Many universities and programs have a flexible timeline for applicants and do not have strict requirements for the amount of time between degrees.
The gap between your Master's degree and PhD application may affect your chances of being accepted, but it is not the only factor that admissions committees consider. Your academic achievements, research experience, and letters of recommendation will also be taken into account.
It is not necessary to explain the reason for the gap in your PhD application, but it can be helpful for the admissions committee to have a better understanding of your circumstances. If the gap was due to personal or health reasons, it may be beneficial to mention it in your application.
Your work experience during the gap period can be valuable in your PhD application, especially if it is related to your field of study. Be sure to highlight any relevant skills or experiences in your application materials, such as your resume or personal statement.
In some cases, it may be possible to transfer credits from your Master's degree towards a PhD program. However, this is ultimately up to the discretion of the university and program you are applying to. It is best to inquire with the admissions office or program coordinator for more information.