It is important to distinguish between serverside and clientside.
Briefly, PHP is used for serverside whereas HTML is for clientside. Now, Bootstrap runs on CSS, and to “simply” integrate Bootstrap and be able to use some of their way of design, click here and follow the instructions.
There is no reason to migrate to Bootstrap unless you are looking for responsiveness for all devices which has many advantages in terms of search engine ranking and more.
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solved Transforming HTML to use Bootstrap [duplicate]