require() is identical to include() except upon failure it will also produce a fatal E_ERROR level error. In other words, it will halt the script whereas include() only emits a warning (E_WARNING) which allows the script to continue.

就是說在失敗的時候，require是會中止php運行的，而include是可以繼續(xù)運行的。
倒底有什么樣的區(qū)別呢？我們帶著這個疑問來一起進入PHP的核心代碼。
下面是一個PHP運行過程的圖（這個圖是出自哪里的？鳥哥畫的？）

補習一下：lex是代碼掃描器，掃描代碼用的，yacc是Yet Another Compiler Compiler，作用是把任何一種代碼的語法轉(zhuǎn)成yacc語法，yacc就是解析器（真TMD繞）。
lex在c下的后綴是*.l yacc是*.y

正題
下面看操作記錄：

cc@cc-laptop:/opt/workspace$ svn checkout http://svn.php.net/repository/php/php-src/branches/PHP_5_3 php-src-5.3
從svn取最新的php源代碼。

開始深入：

cc@cc-laptop:/opt/workspace/php-src-5.3$ find . -type f -name “*.l” -exec grep -Hn “require_once” {} \;
./Zend/zend_language_scanner.l:1093:”require_once” {
尋找lex代碼掃描器文件中出現(xiàn)require_once的地方，zend_language_scanner.l的1093行。
1093 “require_once” {
1094 return T_REQUIRE_ONCE;
1095 }

然后再搜一下T_REQUIRE_ONCE,

cc@cc-laptop:/opt/workspace/php-src-5.3$ find . -type f -name “*.y” -exec grep -Hn “T_INCLUDE” {} \;
./Zend/zend_language_parser.y:52:%left T_INCLUDE T_INCLUDE_ONCE T_EVAL T_REQUIRE T_REQUIRE_ONCE
./Zend/zend_language_parser.y:985: | T_INCLUDE expr { zend_do_include_or_eval(ZEND_INCLUDE, &$$, &$2 TSRMLS_CC); }
./Zend/zend_language_parser.y:986: | T_INCLUDE_ONCE expr { zend_do_include_or_eval(ZEND_INCLUDE_ONCE, &$$, &$2 TSRMLS_CC); }

在985行附近，有這樣一群代碼：

internal_functions_in_yacc:
T_ISSET ‘(‘ isset_variables ‘)' { $$ = $3; }
| T_EMPTY ‘(‘ variable ‘)' { zend_do_isset_or_isempty(ZEND_ISEMPTY, &$$, &$3 TSRMLS_CC); }
| T_INCLUDE expr { zend_do_include_or_eval(ZEND_INCLUDE, &$$, &$2 TSRMLS_CC); }
| T_INCLUDE_ONCE expr { zend_do_include_or_eval(ZEND_INCLUDE_ONCE, &$$, &$2 TSRMLS_CC); }
| T_EVAL ‘(‘ expr ‘)' { zend_do_include_or_eval(ZEND_EVAL, &$$, &$3 TSRMLS_CC); }
| T_REQUIRE expr { zend_do_include_or_eval(ZEND_REQUIRE, &$$, &$2 TSRMLS_CC); }
| T_REQUIRE_ONCE expr { zend_do_include_or_eval(ZEND_REQUIRE_ONCE, &$$, &$2 TSRMLS_CC); }
;

于是乎，我們需要繼續(xù)深入尋找zend_do_include_or_eval，

cc@cc-laptop:/opt/workspace/php-src-5.3$ find . -type f -name “*.c” -exec grep -Hn “zend_do_include_or_eval” {} \;
./Zend/zend_compile.c:4317:void zend_do_include_or_eval(int type, znode *result, const znode *op1 TSRMLS_DC) /* {{{ */

zend_do_include_or_eval中組裝了一個結構體，ZEND_INCLUDE_OR_EVAL。

再在zend_vm_def.h中找到ZEND_VM_HANDLER(73, ZEND_INCLUDE_OR_EVAL, CONST|TMP|VAR|CV, ANY)：
switch (Z_LVAL(opline->op2.u.constant)) {代碼略}

中間關鍵的一句是：
new_op_array = compile_filename(Z_LVAL(opline->op2.u.constant), inc_filename TSRMLS_CC);

在zend_complie.h文件中:
ZEND_API zend_op_array *compile_filename(int type, zval *filename TSRMLS_DC);

這個函數(shù)定義在zend_language_scaner.l文件中，找出最核心的代碼：

if (open_file_for_scanning(file_handle TSRMLS_CC)==FAILURE) {
// require與include的差別：錯誤信息的顯示級別（有bailout和無bailout）
if (type==ZEND_REQUIRE) { //require時
zend_message_dispatcher(ZMSG_FAILED_REQUIRE_FOPEN, file_handle->filename TSRMLS_CC);
zend_bailout();
} else {
zend_message_dispatcher(ZMSG_FAILED_INCLUDE_FOPEN, file_handle->filename TSRMLS_CC);
}
compilation_successful=0;
} else {代碼略}

繼續(xù)追蹤zend_message_dispatcher可以在main/main.c文件中找到php_message_handler_for_zend函數(shù)：

//include輸出錯誤信息時的級別為：E_WARNING
case ZMSG_FAILED_INCLUDE_FOPEN:
php_error_docref(“function.include” TSRMLS_CC, E_WARNING, “Failed opening ‘%s' for inclusion (include_path='%s')”, php_strip_url_passwd((char *) data), STR_PRINT(PG(include_path)));
break;
//require輸出錯誤信息時的級別為：E_COMPILE_ERROR
代碼略

總結
和開頭PHP手冊所說完全一致，require和include的區(qū)別在于，出現(xiàn)錯誤時，一個是error一個是warning。