Why do you need to put a backslash before PHP functions?

backslash.php

<?php
namespace Example;

$a = 23;
\is_null($a);

backslash.php.asm

JIT$/home/test.php: ; (/home/test.php)
  call 0x55b6ce3cc7e0
  mov $EG(exception), %rax
  cmp $0x0, (%rax)
  jnz JIT$$exception_handler
  jmp ZEND_RETURN_SPEC_CONST_LABEL

no-backslash.php

<?php
namespace Example;

$a = 23;
is_null($a);

no-backslash.php.asm

JIT$/home/test.php: ; (/home/test.php)
  call 0x556ad05cc7e0
  mov $EG(exception), %rax
  cmp $0x0, (%rax)
  jnz JIT$$exception_handler
  mov 0x40(%r14), %rdx
  mov 0x8(%rdx), %rax
  test %rax, %rax
  jz .L14
.L1:
  test $0x1, (%rax)
  mov $0x60, %rdi
  jnz .L2
  mov $0x1, %edx
  cmp 0x20(%rax), %edx
  cmova 0x20(%rax), %edx
  sub 0x48(%rax), %edx
  sub 0x40(%rax), %edx
  shl $0x4, %edx
  movsxd %edx, %rdx
  sub %rdx, %rdi
.L2:
  mov $EG(vm_stack_top), %r15
  mov (%r15), %r15
  mov $EG(vm_stack_end), %rdx
  mov (%rdx), %rdx
  sub %r15, %rdx
  cmp %rdi, %rdx
  jb .L15
  mov $EG(vm_stack_top), %rdx
  add %rdi, (%rdx)
  mov $0x0, 0x28(%r15)
  mov %rax, 0x18(%r15)
.L3:
  mov $0x0, 0x20(%r15)
  mov $0x1, 0x2c(%r15)
  mov $0x0, 0x30(%r15)
  mov %r15, 0x8(%r14)
  mov 0x18(%r15), %rax
  test $0x300, (%rax)
  jnz .L16
  cmp $0x0, 0x58(%r14)
  jz .L20
  lea 0x50(%r14), %rdi
  cmp $0xa, 0x8(%rdi)
  jnz .L4
  mov (%rdi), %rdi
  add $0x8, %rdi
.L4:
  mov (%rdi), %rdx
  mov %rdx, 0x50(%r15)
  mov 0x8(%rdi), %eax
  mov %eax, 0x58(%r15)
  test %ah, %ah
  jz .L5
  add $0x1, (%rdx)
.L5:
  mov $0x556ac08d4770, %rax
  mov %rax, (%r14)
  mov $0x0, 0x8(%r14)
  mov %r14, 0x30(%r15)
  mov 0x18(%r15), %rax
  cmp $0x2, (%rax)
  jnz .L10
  mov $0x0, 0x8(%r15)
  mov $0x0, 0x10(%r15)
  mov 0x38(%rax), %rdx
  test $0x1, %rdx
  jz .L6
  mov $CG(map_ptr_base), %rcx
  add (%rcx), %rdx
  mov (%rdx), %rdx
.L6:
  mov %rdx, 0x40(%r15)
  mov $EG(current_execute_data), %rcx
  mov %r15, (%rcx)
  mov %r15, %r14
  mov 0x50(%rax), %r15
  mov 0x20(%rax), %edx
  mov 0x2c(%r14), %ecx
  cmp %edx, %ecx
  jg .L21
  test $0x100, 0x4(%rax)
  jnz .L7
  mov %ecx, %edx
  shl $0x5, %rdx
  add %rdx, %r15
.L7:
  mov 0x48(%rax), %edx
  sub %ecx, %edx
  jle .L9
  shl $0x4, %rcx
  lea 0x50(%r14,%rcx), %rcx
.L8:
  mov $0x0, 0x8(%rcx)
  sub $0x1, %edx
  lea 0x10(%rcx), %rcx
  jnz .L8
.L9:
  jmp (%r15)
.L10:
  test $0x800, 0x4(%rax)
  jnz .L22
.L11:
  mov $EG(current_execute_data), %rcx
  mov %r15, (%rcx)
  mov %rsp, %rsi
  mov $0x1, 0x8(%rsi)
  mov %r15, %rdi
  call 0x48(%rax)
  mov $EG(current_execute_data), %rax
  mov %r14, (%rax)
  mov %r15, %rdi
  mov $zend_jit_vm_stack_free_args_helper, %rax
  call *%rax
  test $0x4, 0x2a(%r15)
  jnz .L23
  mov $EG(vm_stack_top), %rax
  mov %r15, (%rax)
.L12:
  test $0x1, 0x9(%rsp)
  jnz .L24
.L13:
  mov $EG(exception), %rax
  cmp $0x0, (%rax)
  jnz JIT$$icall_throw
  mov $EG(vm_interrupt), %rax
  cmp $0x0, (%rax)
  jnz .L27
  mov $0x556ac08d4790, %r15
  jmp .ENTRY1
.ENTRY1:
  jmp ZEND_RETURN_SPEC_CONST_LABEL
.L14:
  mov $0x556ac08d46d0, %rdi
  lea 0x8(%rdx), %rsi
  mov $zend_jit_find_ns_func_helper, %rax
  call *%rax
  test %rax, %rax
  jnz .L1
  mov %r15, (%r14)
  jmp JIT$$undefined_function
.L15:
  mov %rax, %rsi
  mov $0x556ac08d4730, %rax
  mov %rax, (%r14)
  mov $zend_jit_extend_stack_helper, %rax
  call *%rax
  mov %rax, %r15
  jmp .L3
.L16:
  cmp $0x0, 0x58(%r14)
  jnz .L17
  mov $0x1, 0x58(%r14)
  jmp .L18
.L17:
  cmp $0xa, 0x58(%r14)
  jnz .L18
  mov 0x50(%r14), %rcx
  add $0x1, (%rcx)
  mov %rcx, 0x50(%r15)
  mov $0x10a, 0x58(%r15)
  jmp .L19
.L18:
  call _emalloc_32
  mov $0x2, (%rax)
  mov $0x1a, 0x4(%rax)
  mov $0x0, 0x18(%rax)
  mov 0x50(%r14), %rdx
  mov %rdx, 0x8(%rax)
  mov 0x58(%r14), %ecx
  mov %ecx, 0x10(%rax)
  mov %rax, 0x50(%r14)
  mov $0x10a, 0x58(%r14)
  mov %rax, 0x50(%r15)
  mov $0x10a, 0x58(%r15)
.L19:
  jmp .L5
.L20:
  mov $0x556ac08d4750, %rax
  mov %rax, (%r14)
  mov $0x50, %edi
  mov $zend_jit_undefined_op_helper, %rax
  call *%rax
  mov $0x1, 0x58(%r15)
  test %rax, %rax
  jz JIT$$exception_handler
  jmp .L5
.L21:
  mov $zend_jit_copy_extra_args_helper, %rax
  call *%rax
  mov 0x18(%r14), %rax
  mov 0x2c(%r14), %ecx
  jmp .L7
.L22:
  mov $zend_jit_deprecated_helper, %rax
  call *%rax
  test %al, %al
  mov 0x18(%r15), %rax
  jnz .L11
  jmp JIT$$exception_handler
.L23:
  mov %r15, %rdi
  mov $zend_jit_free_call_frame, %rax
  call *%rax
  jmp .L12
.L24:
  mov (%rsp), %rdi
  sub $0x1, (%rdi)
  jnz .L25
  mov $0x556ac08d4770, %rax
  mov %rax, (%r14)
  call rc_dtor_func
  jmp .L13
.L25:
  cmp $0xa, 0x8(%rsp)
  jnz .L26
  test $0x2, 0x11(%rdi)
  jz .L13
  mov 0x8(%rdi), %rdi
.L26:
  test $0xfffffc10, 0x4(%rdi)
  jnz .L13
  call gc_possible_root
  jmp .L13
.L27:
  mov $0x556ac08d4790, %r15
  jmp JIT$$interrupt_handler