shell_exec command in php not working properly - php

Question

I am trying to execute a C program using the shell_exec command, which needs arguments to be passed. It is working for one input, but not working for others. I tried to run the C program through terminal, it is working for all the inputs.
This is my execprog.php file. I have to give 2 inputs as command line arguments to file. /var/www/project is the path.
$query = "/var/www/project/./a.out /var/www/project/constraints.txt /var/www/project/constraints_keyword.txt /var/www/project/FIB.txt /var/www/project/ANS.txt";
echo $query;
$var = shell_exec($query);
echo $var;

Answer 1

<?php
$query = "/var/www/project/./a.out";
$arguments = array
(
'/var/www/project/constraints.txt',
'/var/www/project/constraints_keyword.txt',
'/var/www/project/FIB.txt',
'/var/www/project/ANS.txt'
);
$string = '';
for($i=0;$i<count($arguments);$i++)
$string.= ' %s';
$command = vsprintf("{$query}{$string}", $arguments);
$var = shell_exec($command);
echo $var;

Answer 2

As you it works on the terminal and not on apache then apache's php.ini file may be disabling the use of shell_exec().
See http://www.php.net/manual/en/ini.core.php#ini.disable-functions
Your apache's php.ini file may look something like
disable_functions=exec,passthru,shell_exec,system,proc_open,popen
Remove shell_exec from this list and restart the web server, although this is a security risk and I don't recommend it.
In general functions such as exec,shell_exec and system are always used to execute the external programs. Even a shell command can also be executed. If these two functions are enabled then a user can enter any command as input and execute into your server. So usually people disable in apache config as disable_functions to secure their site.
It works for me - Here is test run
Sample test c code
[akshay#gold tmp]$ cat test.c
#include<stdio.h>
int main(int args, char *argv[]) {
int i = 0;
for (i = 0; i < args; i++)
printf("Arg[%d] = %s\n",i, argv[i]);
return 0;
}
Compile
[akshay#gold tmp]$ gcc test.c
Sample php script
[akshay#gold tmp]$ cat test.php
<?php
$query = "/tmp/./a.out /var/www/project/constraints.txt /var/www/project/constraints_keyword.txt /var/www/project/FIB.txt /var/www/project/ANS.txt";
$var = shell_exec($query);
echo $var;
?>
Execution and output
[akshay#gold tmp]$ php test.php
Arg[0] = /tmp/./a.out
Arg[1] = /var/www/project/constraints.txt
Arg[2] = /var/www/project/constraints_keyword.txt
Arg[3] = /var/www/project/FIB.txt
Arg[4] = /var/www/project/ANS.txt