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copied!<p>The obvious solution would be to call <a href="http://us2.php.net/manual/en/function.file-exists.php" rel="nofollow noreferrer">file_exists</a> to check to see if the file exists, but doing that could cause a race condition. There is always the possibility that the other file will be created in between when you call <a href="http://us2.php.net/manual/en/function.file-exists.php" rel="nofollow noreferrer">file_exists</a> and when you call <a href="http://us2.php.net/manual/en/function.copy.php" rel="nofollow noreferrer">copy</a>. The only safe way to check if the file exists is to use <a href="http://us2.php.net/manual/en/function.fopen.php" rel="nofollow noreferrer">fopen</a>.</p> <p>When you call <a href="http://us2.php.net/manual/en/function.fopen.php" rel="nofollow noreferrer">fopen</a>, set the mode to 'x'. This tells <a href="http://us2.php.net/manual/en/function.fopen.php" rel="nofollow noreferrer">fopen</a> to create the file, but only if it doesn't exist. If it exists, <a href="http://us2.php.net/manual/en/function.fopen.php" rel="nofollow noreferrer">fopen</a> will fail, and you'll know that you couldn't create the file. If it succeeds, you will have a created a file at the destination that you can safely copy over. Sample code is below:</p> <pre><code>// The PHP copy function blindly copies over existing files. We don't wish // this to happen, so we have to perform the copy a bit differently. The // only safe way to ensure we don't overwrite an existing file is to call // fopen in create-only mode (mode 'x'). If it succeeds, the file did not // exist before, and we've successfully created it, meaning we own the // file. After that, we can safely copy over our own file. $filename = 'sourcefile.txt' $copyname = 'sourcefile_copy.txt' if ($file = @fopen($copyname, 'x')) { // We've successfully created a file, so it's ours. We'll close // our handle. if (!@fclose($file)) { // There was some problem with our file handle. return false; } // Now we copy over the file we created. if (!@copy($filename, $copyname)) { // The copy failed, even though we own the file, so we'll clean // up by itrying to remove the file and report failure. unlink($copyname); return false; } return true; } </code></pre>

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