Error with php code, error displayed Undefined Offset - php

Question

this line of code keeps getting me undefined offset: on line 9. I have no idea whats wrong with it, the codes runs smoothly but this part of the code is getting me undefined offset. i believe its something with $i
for( $i = 0; $lines[$i]; $i++ ) /** LINE 9 **/
{
if( $_POST['Aut'] == rtrim($lines[$i]) )
{
fwrite($file, $_POST['addition']."\n\t");
fwrite($file, $_POST['Aut'].PHP_EOL);
}
else
{
fwrite($fd,$lines[$i]);
}

Answer 1

for( $i = 0; $lines[$i]; $i++ )
When does the loop end? One iteration too late. It has to be
for( $i = 0; $i<count($lines); $i++ )
^
For Loop Syntax

Answer 2

for( $i = 0; **$lines[$i]**; $i++ )
Second step of for loop should be condition which is missing here

Answer 3

Try with -
foreach($lines as $line) /** LINE 9 **/
{
if( $_POST['Aut'] == rtrim($line) )
{
fwrite($file, $_POST['addition']."\n\t");
fwrite($file, $_POST['Aut'].PHP_EOL);
}
else
{
fwrite($fd,$line);
}
}

Answer 4

You should define the $lines array before using in the for loop.
$lines[] = array(1,2,3,4,5);
for( $i = 0; $lines[$i]; $i++ ) /** LINE 9 **/
{
print 'hai'; print PHP_EOL;
}
here, the "hai" would print 5 times from $lines[0] to $lines[4]. and the next it will return the same error, because there is no data on the $lines[5]

Answer 5

you can simply use the foreach for this purpose
foreach($lines as $key=>$line)
{
/* your code here */
}
if you want to do this with for loop try this one
for( $i = 0; $i<count($lines); $i++ ) /** LINE 9 **/
{
if( $_POST['Aut'] == rtrim($lines[$i]) )
{
fwrite($file, $_POST['addition']."\n\t");
fwrite($file, $_POST['Aut'].PHP_EOL);
}
else
{
fwrite($fd,$lines[$i]);
}