Replace a string in a file?

Hi, I have a file (test.txt) that looks like:

wfawfawaf
[COLOR=Red]Unique Text
bogfwg
wfawgw
gwa
wa
gagwgawwga
ggawgwa

wag
gwawgwagwagwag
gagwgwa
End Unique Text[/COLOR]

wfanwfa
gawfwfafwa
gaefwawfa

Code (markup):

I would to replace the red text with $testvar

How do I do that?

I could just read out the file using file_get_contents()
then change the content with str_replace()
then put it back with file_put_contents()
Is that the best way?
problem is, the file can get big, and it sounds like overkill

 

how big is your file and you have php5 (to use file_put_contents) ?

 

well the size can change.
But i have decided to just use the method I described in the first post.

 

Read in blocks, especially if the text you are replacing only occurs once.

You can then optimise by starting off at a certain point if you know roughly where the unique text is located etc.