在linux中脚本执行结束后，php exec或shell_exec不会终止其进程

I am trying to check for syntax error of a php file using shell commands, its working fine in Windows (WAMP) but on linux the process created by shell commands exec/shell_exec/popen etc never terminates thus causing apache to hang while this process gets forcefully killed by me. also no output is generated after killing the process.I am trying to check for syntax error of a php