The strip_tags() function searches for and removes an unlimited number of tags from a string. This function accepts and requires one parameter, and can remove tags with or without attributes.
echo(remove_tags); //this should output "This piece of string is bold but this isn't"
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If you're familiar with PHP, you should have no trouble understanding how this function works. For a greater understanding, please test this function in your Web Console.
Additionally, you can play around with the demo below to see this function in action on a live webpage! Enter some text with at least one word covered in HTML tags, and then click "Remove tags" to see the text alerted without the tags.