Search and replace string in PHP

Search and replace string in php.
Search and replacing string is not difficult using the function str_replace();
I have to search a string (actually an html content which i extracted from a file using file_get_content()), string to was src=" and replace with src="http://path/to/the/image/folder/ and i acheived this by simply str_replace('src="','src="http://path/to/the/image/folder/',$str);

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