Are there existing functions in geshi that allow for selecting and highlighting only specific code within a block of text?

Yes, there are existing functions in GeSHi that allow for selecting and highlighting only specific code within a block of text. One way to achieve this is by using the `set_code` and `parse_code` functions provided by GeSHi. By setting the code to be highlighted using `set_code` and then parsing only that specific code using `parse_code`, you can highlight only the selected code within a larger block of text.

// Create a new GeSHi object
$geshi = new GeSHi();

// Set the code to be highlighted
$geshi->set_code($full_code_block);

// Specify the language
$geshi->set_language('php');

// Parse and output only the specific code within the block
echo $geshi->parse_code($selected_code);