I need to find a *.jpg name in a text document. For example I have a folder with pictures file1, file2 , file3 and a text document with file1, file, file3 each on a new line. I need to write near each *.jpg the text from the file, but first I need to find the corresponding row in the text document.

 <?php
 $arrayF = explode("
", file_get_contents('myTxt.txt'));
 // arrayF should the the array with each text line from the txt file.
  while(($file = readdir($opendir)) !== FALSE)
       {
           if($file!="." && $file!="..")
           {
                $string=$file;
                $arrayS = explode('.', $string);//get the name only without .jpg extension
                $search=$arrayS[0];

        $key = array_search($search, $arrayF);

        echo $key;
       }

Below is an example to iterate through a directory using DirectoryIterator which provides viewing contents of filesystem directories.

Example:

foreach(new DirectoryIterator($dir) as $file_info)
{
    // sleep a micro bit (up to 1/8th second)
    usleep(rand(0, 125000));

    // disregard hidden, empty, invalid name files
    if($file_info == null or $file_info->getPathname() == '' 
                          or $file_info->isDot())
    {
        continue;
    }

    // check if its a file - log otherwise
    // code omitted

    // get filename and filepath
    $filepath = $file_info->getPathname();
    $filename = $file_info->getFilename();

   // my answer to read file linked here to avoid duplication
   // below "read/parse file into key value pairs"
}

read/parse file into key value pairs

To elaborate new DirectoryIterator( path to dir ) provides an iterator which can be also written as:

$iterator = new DirectoryIterator($directory);
foreach ($iterator as $fileinfo) {
  // do stuffs
}

Hope this helps, indirectly.

$string=file_get_contents($file);

file_get_contents

Or if you want it line by line, just open, read and close

$fp = fopen($file, 'r');
while ($line = fread($fp)) {
//do comparision stuff
}
fclose ($fp);