echo writes successfully on 8 bytes input but failed for others - php

Question

Thought a lot but could not find a suitable title ... so also suggest on title.
Also I am not showing HTML code to avoid making post larger. With PHP variables names and part of figure shown, its easy to understand the situation.
Actual problem is described here -
Here is a code snippet which is just a small part of a large code. The confusion is b/w two situations. Both are mentioned below. -
PHP Code :
while($npacket>=1) {
$count=0;
$out_file=fopen("packet.txt","w");
while($count<$payload_length ) {
$c=fgetc($file);
fputs($out_file,$c);
$count++;
}
$data_file=`cat packet.txt`;
if($datatyp=="ascii")
$fpayload="-f packet.txt";
else if($datatyp=="hex")
$fpayload="-d0x$data_file";
$random="";
if( $ip=="tcp" || $ip=="udp" )
exec("sendip -v -p ipv6 $fpayload $random $adv_cont -6s $sip $optext -p $proto $cont -$s $sapp_port -$d $dapp_port $dip ");
else {
Problem is Here ....
echo "this is $fpayload it";
exec(" echo sendip -v -p ipv6 $fpayload $random $adv_cont -6s $sip $optext -p icmp $icmp_cont $dip > sample ");
}
usleep("$igap");
if(!$flag)
$npacket--;
}
fclose($out_file);
fclose($file);
}
For 1 packet and payload specified as a input,
1) Input : 616263646566
Output :
$ cat payload.txt
616263646566
$ cat packet.txt
616263646566
$ cat sample
$
2) Input : 6162636465666768
Output :
$ cat payload.txt
6162636465666768
$ cat packet.txt
6162636465666768
$ cat sample
sendip -v -p ipv6 -d0x6162636465666768 -6s 2001::100 -p icmp 2001::200
Please help me find out where does exactly problem lies ?
EDIT:
Permission of files :
$ ls -l payload.txt packet.txt sample
-rwxrwxrwx 1 udit udit 13 Nov 17 20:57 packet.txt
-rwxrwxrwx 1 udit udit 13 Nov 17 20:57 payload.txt
-rwxrwxrwx 1 udit udit 0 Nov 17 20:57 sample

Answer 1

Problem was actually presence of whitespaces and null characters at the end of $fpayload variable. Just needed to add one regular expression syntax in else part.
.........
else if($datatyp=="hex") {
$fpayload="-d0x$data_file";
$fpayload=preg_replace('/\s+/', '', $fpayload);
}
....