Replacing file content in PHP

You can do:

$file = 'filename';
file_put_contents($file,str_replace('find','replace',file_get_contents($file)));

@codaddict's answer is quite sufficent for small files (and would be how I would implement it if the size of the file was under a MiB). However it will eat up a ton of memory, and as such you should be careful when reading large files.

If you want a much more memory friendly version, you could use stream filters...

class ReplaceText_filter extends php_user_filter {
    protected $search = '';
    protected $replace = '';

    public function filter($in, $out, &$consumed, $closing) {
        while ($bucket = stream_bucket_make_writable($in)) {
            $bucket->data = str_replace(
                $this->search, 
                $this->replace, 
                $bucket->data
            );
            $consumed += $bucket->datalen;
            stream_bucket_append($out, $bucket);
        }
        return PSFS_PASS_ON;
    }

    public function onCreate() {
        if (strpos($this->filtername, '.') === false) return false;
        list ($name, $arguments) = explode('.', $this->filtername, 2);
        $replace = '';
        $search = $arguments;
        if (strpos($arguments, '|') !== false) {
            list ($search, $replace) = explode('|', $arguments, 2);
        }
        if (strpos($search, ',') !== false) {
            $search = explode(',', $search);
        }
        if (strpos($replace, ',') !== false) {
            $search = explode(',', $replace);
        }
        $this->search = $search;
        $this->replace = $replace;
    }
}
stream_filter_register('replacetext.*', 'ReplaceText_Filter');

So, then you can append an arbitrary stream filter. The filter's name determines the arguments:

$search = 'foo';
$replace = 'bar';
$name = 'replacetext.'.$search.'|'.$replace;
stream_filter_append($stream, $name);

or for arrays,

$search = array('foo', 'bar');
$replace = array('bar', 'baz');
$name = 'replacetext.'.implode(',', $search).'|'.implode(',', $replace);
stream_filter_append($stream, $name);

Obviously this is a really simple example (and doesn't do a lot of error checking), but it allows you to do something like this:

$f1 = fopen('mysourcefile', 'r');
$f2 = fopen('mytmpfile', 'w');
$search = array('foo', 'bar');
$replace = array('bar', 'baz');
$name = 'replacetext.'.implode(',', $search).'|'.implode(',', $replace);
stream_filter_append($f1, $name);
stream_copy_to_stream($f1, $f2);
fclose($f1);
fclose($f2);
rename('mytmpfile', 'mysourcefile');

And that will keep memory usage very low while processing potentially huge (GiB or TiB) files...

Oh, and the other cool thing, is it can inline edit differing stream types. What I mean by that is that you can read from a HTTP stream, edit inline, and write to a file stream. It's quite powerful (as you can chain these filters)...