[ <?php echo 'Hello <I>World</I>'; ?> ]   follows...
[ Hello World ]
(above, 'echo' is a PHP command, not the unix 'echo')
variables ($x etc), commands, ';', strings, concatenate '.' (also '.=') ...
3=1+2 -- <?php $x=1; $y=2; $z=$x+$y; echo $z . "=" . $x."+".$y ?>
escapeshellarg(...), escapeshellcmd(...)
$str = abc'singlequoted'def.ghi/j!k;m@np&qs|tu>vw
escapeshellarg($str) = 'abc'\''singlequoted'\''def.ghi/j!k;m@np&qs|tu>vw'
escapeshellcmd($str) = abc'singlequoted'def.ghi/j!k\;m@np\&qs\|tu\>vw
[exec, system, passthru], ...
[ ] -- [ <?php exec( "./hello" ); ?> ]
"execute an external program".
(note, [ hello world from hello.c ] -- [ <?php echo exec( "./hello" ); ?> ])
"Warning: When allowing user-supplied data to be passed to this function, use escapeshellarg() or escapeshellcmd() to ensure that users cannot trick the system into executing arbitrary commands."
[ hello world from hello.c ] -- [<?php system( "./hello" ); ?>]
"execute an external program and display the output." And, "tries to automatically flush the web server's output buffer after each line of output if PHP is running as a server module."
Warning as above.
[ hello world from hello.c ] -- [<?php passthru( "./hello" ); ?>]
"execute an external program and display raw output".
Warning as above.
[eg.c] : [ -- begin eg.c --
argv[0]="./eg" argv[1]="abc" argv[2]="pqr" argv[3]="xyz"
-- done eg.c --
]
[Forms]
<form action="welcome_post.php" method="post"> ...
Name: field2: ...etc.
→ .../welcome_post.php

<form action="welcome_get.php" method="get"> ...
Name: field2:
→ .../welcome_get.php?name=abc&field2=def
get: eg_get.php → eg.c ...
field1: field2:
post: eg_post.php → eg.c ...

Also see [phpinfo()]. (Search for "_max_", "max" and "limit".)