Writing to files (db related) ish

Hi All I know how to open, read & write to files using Php. But how can you write some content to a particular place in a file, Not the start or end but say the middle some where. Example: Basic html file <html> <head> <title></title> </head> <body> Write my stuff here!!! </body> </html> This eventually will relate to a db so I hope its not completely of topic. Sorry if it is. Web resources, examples, tutorials gratefully received. Thanks in advance Dave C