$p = 123456789101112
 $pyscript = 'C:\Users\python\teste.py';
 $python = 'C:\Python34\python.exe';
 $run = shell_exec("$python $pyscript $p");

import sys
print(sys.argv[1])

precision 14

$p = '123456789111315171921'; // note as aspas aqui
 $pyscript = 'C:\Users\python\teste.py';
 $python = 'C:\Python34\python.exe';
 $run = shell_exec("$python $pyscript $p");