Replace a charachter in a string while keeping the rest of the string

If I am pulling a variable name from a dayabase on the sserver, and I want to extract any spaces in the name and replace it with underscores, how would I go about doing it. I looked at ereg_replace() but that will not save the rest of the string.

an example:

I want to make the word “Foo Bar” look like “Foo_Bar”

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