How to change part of URL with PHP function?

This is the scenario:

[1] I am using Wordpress

[2] I use a plugin that pulls products from a online seller and displays it on my website.

[3] My website is on SSL and the source website has both SSL & non-SSL

[4] The image sources can be both:

http ://somewebsite.com/folder/someimage.jpg

OR

https ://securewebsite.com/folder/someimage.jpg

[5] I want to use a function in my Wordpress theme (or in the plugin itself) that will change part of the image URL before the webpage is displayed.

change the first part of the image URL :

***http ://somewebsite.com/folder/***someimage.jpg

and replace it with

***https ://securewebsite.com/folder/***someimage.jpg

via Chebli Mohamed