invalid argument when imploding in php - php

Question

Im getting invalid argument error when running the following code. Im trying to change the value of a line in the $info array, then implode it, implode its parent array, and then save the whole shebang back to whence it came.
$rowpre = $_GET['row'];
$newfieldvalue = $_GET['nfv'];
$row = --$rowpre;
$data = file_get_contents("temp.php");
$csvpre = explode("###", $data);
$i = 0;
foreach ( $csvpre AS $key => $value){
$i++;
if($i = $row){
$info = explode("%%", $value);
$info[$target] = $newfieldvalue;
$presave = implode("%%", $info);
}
}
$save = implode("###", $presave);
$fh = fopen("temp.php", 'w') or die("can't open file");
fwrite($fh, $save);
fclose($fh);
update below
$rowpre = $_GET['row'];
$newfieldvalue = $_GET['nfv'];
$target = $_GET['target'];
$row = --$rowpre;
$data = file_get_contents("temp.php");
$csvpre = explode("###", $data);
$i = 0;
foreach ( $csvpre AS $key => $value){
$i++;
if($i == $row){
$info = explode("%%", $value);
$info[$target] = $newfieldvalue;
$csvpre[$key] = implode("%%", $info);
}
}
$save = implode("###", $csvpre);
$fh = fopen("temp.php", 'w') or die("can't open file");
fwrite($fh, $save);
fclose($fh);
Target is the field within the selected Row that i wish to update with the newfieldvalue data.

Answer 1

$save = implode("###", $presave);
At that point, $presave is a string, and should be an array to work with implode. Create an array where you push the $presave-values, and implode that one.

Answer 2

$presave contains the last processed line (i.e. a string) and implode expects an array. To store the line back in the original array, change:
$presave = implode("%%", $info);
to:
$csvpre[$key] = implode("%%", $info);
And to convert the whole CSV array into a string, change:
$save = implode("###", $presave);
to:
$save = implode("###", $csvpre);
And one more problem:
if($i = $row){
should be:
if($i == $row){
because you want to compare the variables, not assign $i.