First of all, thank you for replying me.
I have put the file into the directory where my PHP file is. When I use "passthru" it might know that it is there (but the fact is that the web page lasts a lot of time to appear, and finally appears a message telling me that the time have been exceeded.
If it can help, I must say that the executable file is a Pascal one. I have tried with a complied C program and it works perfectly! One person here has told me that perhaps it can be because Pascal is old and works with 16 bits, and that using FreePascal the problem will be solved. Well, I have compiled a simple Pascal program and the problem is still there.