function file_intersperse_contents($szFilename, $szInject, $szFind) {
        
    // Check if file exist
    if(file_exists($szFilename)) {
            
        // If file exist
        // Get the content
        $szContent = file_get_contents($szFilename);
        
        // Replace the text
        $szReplace = str_replace($szFind, $szInject, $szContent);
        
        // Open the file
        $szFile = fopen($szFilename, 'w+');
        
        // Write the text into it
        $bWrite = fwrite($szFile, $szReplace);

        if($bWrite) {
            
            // If success, close file and return true
            fclose($szFile);
            return true;
            
        }
        
        // If failure, close file and return false
        fclose($szFile);
        return false;

    }

    else {

        die('File' . $szFilename . 'does not exist!');

    }

}

Change line in a file

PHP