接下来我会从源码的角度来观察这些现象

看一段简单的代码

$a = 1;

echo "aaaa\n";

然后查到赋值对应的opcode是 ZEND_ASSIGN_SPEC_CV_CONST_HANDLER，在zend_vm_execute.h中找到对应的实现

static int ZEND_FASTCALL ZEND_ASSIGN_SPEC_CV_CONST_HANDLER(ZEND_OPCODE_HANDLER_ARGS)

{

USE_OPLINE

zval *value;

zval **variable_ptr_ptr;

SAVE_OPLINE();

value = opline->op2.zv;

variable_ptr_ptr = _get_zval_ptr_ptr_cv_BP_VAR_W(execute_data, opline->op1.var TSRMLS_CC);

if (IS_CV == IS_VAR && UNEXPECTED(variable_ptr_ptr == NULL)) {

if (zend_assign_to_string_offset(&EX_T(opline->op1.var), value, IS_CONST TSRMLS_CC)) {

if (RETURN_VALUE_USED(opline)) {

zval *retval;

ALLOC_ZVAL(retval);

ZVAL_STRINGL(retval, Z_STRVAL_P(EX_T(opline->op1.var).str_offset.str)+EX_T(opline->op1.var).str_offset.offset, 1, 1);

INIT_PZVAL(retval);

EX_T(opline->result.var).var.ptr = retval;

}

} else if (RETURN_VALUE_USED(opline)) {

PZVAL_LOCK(&EG(uninitialized_zval));

EX_T(opline->result.var).var.ptr = &EG(uninitialized_zval);

}

} else if (IS_CV == IS_VAR && UNEXPECTED(*variable_ptr_ptr == &EG(error_zval))) {

if (0) {

zval_dtor(value);

}

if (RETURN_VALUE_USED(opline)) {

PZVAL_LOCK(&EG(uninitialized_zval));

EX_T(opline->result.var).var.ptr = &EG(uninitialized_zval);

}

} else {

if (IS_CONST == IS_TMP_VAR) {

value = zend_assign_tmp_to_variable(variable_ptr_ptr, value TSRMLS_CC);

} else if (IS_CONST == IS_CONST) {

value = zend_assign_const_to_variable(variable_ptr_ptr, value TSRMLS_CC);

} else {

value = zend_assign_to_variable(variable_ptr_ptr, value TSRMLS_CC);

}

if (RETURN_VALUE_USED(opline)) {

PZVAL_LOCK(value);

EX_T(opline->result.var).var.ptr = value;

}

}

/* zend_assign_to_variable() always takes care of op2, never free it! */

CHECK_EXCEPTION();

ZEND_VM_NEXT_OPCODE();

}