Our website was initially developed by PHP. However we are trying to convert it into WordPress. Will the SEO be effected?
Maybe. 
Without more information, who can say?
I presume by “SEO” you mean search engine ranking, which is not affected by whether your site runs on plain HTML, PHP, or a CMS such as Wordpress. What does affect it is high quality content and good page structure. Page load time may be another, lesser, factor. So if the move improves page structure or load time, or you add new content, it might help your rankings. If the page structure is less good, or the load time increases (Wordpress is notoriously slow in many instances), then it might have a detrimental effect.
You might find this article from Google helpful for moving a site with minimal disruption: https://developers.google.com/search/docs/advanced/crawling/site-move-with-url-changes?hl=en
2 Likes
Yes for sure if things are not arranged properly.
You have to carefully take care of the following things,
- Keep the same URL pattern
- Configure redirects in WordPress using plugins
- Don’t let change headings immediately (slowly improve them)
- Monitor performance on search console