Senior LevelPHP 8+
How is the Zend Engine zval container structured in C memory across PHP 7 and PHP 8?
❓ Interview Prompt: Explain Zend Engine zval C memory layout, value unions, type info flags, refcount pointers, and garbage collection metadata.
Inside the C source code of the Zend Engine, every PHP variable is represented by a 'zval' (Zend Value) structure.
PHP 7/8 zval Memory Optimization:
In PHP 5, a 'zval' was a 24-byte heap-allocated structure. In PHP 7+, the 'zval' was redesigned as a 16-byte stack-allocated structure:
1
8-Byte Value Union ('zend_value'): Stores scalar values (integers, doubles) directly inline in the struct! For complex types (strings, arrays, objects), it stores a C pointer to heap structures ('zend_string', 'zend_array').
2
8-Byte Type Info & Metadata: Stores type flags, reference counts ('refcount'), and garbage collection cycle colors.
Code Implementation Example
php
/* Conceptual C struct representation of Zend Engine zval container */
struct _zval_struct {
zend_value value; // 8-byte union storing scalar value OR pointer to heap
union {
struct {
zend_uchar type; // IS_NULL, IS_LONG, IS_DOUBLE, IS_STRING, etc.
zend_uchar type_flags;
uint16_t extra;
} v;
uint32_t type_info;
} u1;
union {
uint32_t next; // Hash collision chain / GC info
} u2;
};💡Conceptual C struct layout of Zend Engine 16-byte zval variable container.
🎯 Interview Key Takeaway
In PHP 7/8, zval is a 16-byte stack-allocated structure storing scalars inline and pointers to heap structures for complex types.
#Zend Engine#zval#C Internals#Senior
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