从源码角度看 PHP 字符串类型转换

<?php
$var = false;echo '$var:', $var, "\r\n";
$var = null;echo '$var:', $var, "\r\n";
$var = true;echo '$var:', $var, "\r\n";
$var = 1;echo '$var:', $var, "\r\n";
$var = 1.1;echo '$var:', $var, "\r\n";$var1 = &$var;echo '$var:', $var1, "\r\n";
$var1 = 'string111';echo '$var:', $var1, "\r\n";
$var = [1, 2, 3];echo '$var:', $var, "\r\n";
class Obj{    private $field;}
$var = new Obj();echo '$var:', $var, "\r\n";

$ php string.php$var:$var:$var:1$var:1$var:1.1$var:1.1$var:string111$var:Notice: Array to string conversion in C:\Users\Administrator\Desktop\string.phpon line 24
Call Stack:    0.0010     405568   1. {main}() C:\Users\Administrator\Desktop\string.php:0
Array$var:Catchable fatal error: Object of class Obj could not be converted to string in C:\Users\Administrator\Desktop\string.php on line 32
Call Stack:    0.0010     405568   1. {main}() C:\Users\Administrator\Desktop\string.php:0

ZEND_API zend_string* ZEND_FASTCALL _zval_get_string_func(zval *op) /* {{{ */{try_again:  switch (Z_TYPE_P(op)) {    case IS_UNDEF:    case IS_NULL:    case IS_FALSE:      return ZSTR_EMPTY_ALLOC();    case IS_TRUE:      return ZSTR_CHAR('1');    case IS_RESOURCE: {      char buf[sizeof("Resource id #") + MAX_LENGTH_OF_LONG];      int len;
      len = snprintf(buf, sizeof(buf), "Resource id #" ZEND_LONG_FMT, (zend_long)Z_RES_HANDLE_P(op));      return zend_string_init(buf, len, 0);    }    case IS_LONG: {      return zend_long_to_str(Z_LVAL_P(op));    }    case IS_DOUBLE: {      return zend_strpprintf(0, "%.*G", (int) EG(precision), Z_DVAL_P(op));    }    case IS_ARRAY:      zend_error(E_NOTICE, "Array to string conversion");      return zend_string_init("Array", sizeof("Array")-1, 0);    case IS_OBJECT: {      zval tmp;      if (Z_OBJ_HT_P(op)->cast_object) {        if (Z_OBJ_HT_P(op)->cast_object(op, &tmp, IS_STRING) == SUCCESS) {          return Z_STR(tmp);        }      } else if (Z_OBJ_HT_P(op)->get) {        zval *z = Z_OBJ_HT_P(op)->get(op, &tmp);        if (Z_TYPE_P(z) != IS_OBJECT) {          zend_string *str = zval_get_string(z);          zval_ptr_dtor(z);          return str;        }        zval_ptr_dtor(z);      }      zend_error(EG(exception) ? E_ERROR : E_RECOVERABLE_ERROR, "Object of class %s could not be converted to string", ZSTR_VAL(Z_OBJCE_P(op)->name));      return ZSTR_EMPTY_ALLOC();    }    case IS_REFERENCE:      op = Z_REFVAL_P(op);      goto try_again;    case IS_STRING:      return zend_string_copy(Z_STR_P(op));    EMPTY_SWITCH_DEFAULT_CASE()  }  return NULL;}/* }}} */